We ask acc to Rebbe Yose who holds we collect a rabbinical obligation, what is difference between dorayso and drabanan,
Why don't we say that according to him Mepich shevuah is the difference
Shalom Moshe,
If I understand your point, you are asking why, once the Gemara adopts Rebbi Yosi, who allows collection even for a rabbinic obligation, we cannot reuse the earlier difference of Meipach Shevu'ah.
It seems the Gemara is working cumulatively. That is, if Mar bar Rav Ashi accepts Rebbi Yosi, we must account for his view that Meipach Shevu'ah applies to d'Oraisa cases, while collection from property extends even to d'Rabanan cases. Consequently, we still need a third Nafka Minah. In other words, the Gemara continues searching for an additional practical difference, precisely because -- under Rebbi Yosi's framework -- both earlier distinctions no longer separate the two categories.
Please correct me if I have misunderstood your question.
Kol Tuv,
Aharon Steiner