Some wonder why Rashi only rarely quotes the Yerushalmi while Tosfos frequently quotes it. This morning I found an intriguing answer offered in the name of Rav Chaim Volozhiner.
Kerem Yehoshua By Rabbi Yehoshua Cohen, Chapter 10
For all disagreements between Rashi and Tosfos, we will find that the simple explanation of the Gemara at hand is according to Rashi and not Tosfos...Tosfos usually attacks Rashi by showing his interpretation contradicts some Gemara or passage...so Tosfos offers a different explanation of the Gemara, which does not fit so well with the plain meaning of the text....
Did Rashi fail to see those difficulties which were raised by Rabbeinu Tam, Rabbeinu Yitzchak and others based on other Gemaras or logic?...The Gaon Rabbi Chaim of Volozhin showed that...there is only one dispute - a dispute about convention, tradition and method - from which stem the vast majority of disagreements.
We see many places where the Gemara shows two parts of a single Mishna or braisa seem to contradict each other....If no simple explanation exists, there are two ways to resolve the problem - although neither one is completely satisfactory. One way is to conclude that the mishna or braisa contains the conflicting opinions of two different tanaim...tavra...or the beginning is the opinion of Rabi Yishmael and the end is the opinion of Rabbi Akiva....The other way is to conclude that both parts of the mishna do indeed reflect the opinion of a single Tanna, but each part refers to a different case (see BM 41a, Sanhedrin 62b, Kiddushin 63b, Shabbos 86a).
A careful examination shows Rabbi Zeira, Rabbi Elazar and Rabbah almost always resolve the problem by saying "it is split - whoever taught this did not teach that." On the other hand Rava always takes the other approach, attributing the mishna to a single author, but concluding that it deals with two different topics.
(Similarly...Rav Nachman Bar Yitzchak takes a third approach called litzdadim ketani...but we will not elaborate).
Why did Rava not use the other approach and why didn't Rabbi Zeira et. al. use Rava's approach?
The Gaon Rabbi Chaim of Volozhin explained that what divides the two groups is the tradition which they received from their teachers. Rabbi Zeira, Rabbi Elazar and Rabbah had a tradition that Rabbi Yehuda Hanasi wrote the mishnayos in clear, simple language, to be understood by everyone, not only his greatest disciples. This also applies to braisos....Thus they could not accept Rava's explanation that the beginning and end of a mishna are speaking of two separate topics. If it were so, then according to their tradition, Rabbi Yehuda Hanasi would have said so explicitly. If he did not say so, it must be that there is no change of subject. Thus Rabbi Zeira, Rabbi Elazar and Rabbah have no choice but to say Tavra....
Rava sees things differently, for he had a different tradition....Occasionally he would write a complex mishna...to sharpen the minds of his students....He assumed his students' hard work would yield them the correct understanding of the mishna and bring them to the conclusion that the end of the mishna dealt with a different subject....Rava maintains Rabbi Yehuda Hanasi did this on purpose....
In light of all this, the gaon Rabbi Chaim of Volozhin elegantly explains the disagreements between Rashi and Tosfos. Just as Rabbi Zeira, Rabbi Elazar and Rabbah disagreed with Rav over how Rabbi Yehuda Hanasi wrote the Mishna, so Rashi and Tosfos disagreed how Rav Ashi and Ravina transmitted the Gemara.
According to the tradition Rashi received from his teachers, when Rav Ashi and Ravina compiled the Talmud they wrote in such a way than anyone could understand their words, with no need to employ hair-splitting logic or search for some restricted case (okimta) to which the Gemara might apply. Now, when Rashi encountered a difficult passage, he could explain the Gemara according to its simple meaning, even though that meaning might contradict some other Gemara or might involve certain logical problems, or he could depart from the simple meaning of the Gemara and assume the Gemara referred to some special, restricted case. Faced with such a choice, and based on the tradition he recieved form his teachers, Rashi could not by any means adopt the second alternative. He was forced to adopt the first one, and interpret the meaning according to its simple meaning.
However, Rabbeinu Tam, Rabbeinu Yitzchak and the other Tosafists had a different tradition. They were taught that occasionally Rav Ashi and Ravina would teach a law without mentioning that it applied only to a certain restricted case. They did this in order to sharpen their students and force them to work harder....After much hard work and deep thoughts, the students on their own accord would come to the proper conclusion: the Talmud is not to be understood according to its simple meaning and applied to all cases, but refers only to one particular and unstated case....
(form here to end is verbatim)
I would like to take this insight one step further and apply it to a fact noted above. In the more than 500 disagreements between Rashi and Tosfos which are connected to the Jerusalem Talmud, we see that Rashi never adopts the viewpoint of the Jerusalem Talmud, while Tosfos always does. This cannot be merely because Rashi lacked certain parts of the Yerushalmi while the Tosafists, who lived 100 years later, had these parts. Rather, in my humble opinion, when we clsoely examine these disagreements between Rashi and Tosfos, we will find that in the cases where Rashi interprets contrary to the Jerusalem Talmud, the Jerusalem Talmud cannot be easily reconciled with with the wording of the Babylonian Talmud. Thus Rashi simply remained true to his tradition that Ravina and Rav Ashi, the compilers of the Babylonian Talmud, always wrote in such a way that anyone could understand their words. Based on this tradition, he maintained that if the Babylonian Talmud had agreed with the Jerusalem Talmud, Rav Ashi and Ravina would have written their text to express this agreement more clearly. They would have stated the law in phraseology parallel to that of the Jerusalem Talmud; or they would have restricted the law to the same specific case to which the Jerusalem Talmud restricted it. If they did not do so, this shows (according to Rashi) that they disagreed with the Jerusalem Talmud.
The Tosafists' opinion, however, maintained that not evey law in the Babylonian Talmud is stated in the clearest possible way. Rather, Rav Ashi and Ravina, in order to sharpen their students, would occasionally teach a law without mentioning that it applied only to a certain restricted case. Wherever there is an apparent disagreement between the Babylonian Talmud and Jerusalem Talmud - for example, where the Babylonian Talmud presents a law as being broadly applicable, while the Jerusalem Talmud presents it as applying to one restircted case - Tosfos reasons as follows: why should we assume the Babylonian Talmud disagrees with the Jerusalem Talmud? Let us assume instead that the Babylonian Talmud agrees that the law applies only to one restricted case, but this is one of those places where Rav Ashi and Ravina deliberately did not mention it. Although this involves interpreting the Babylonian Talmud in a way that does not fit the plain meaning of the words, Tosfos does not see this as far-fetched. Tosfos maintains that such occasional discrepancies were intentional. According to Tosfos, it is better to interpret the Babylonian Talmud in this way than to assume a disagreement between it and the Jerusalem Talmud.