First of all, let us try and understand why in most places the Halachah follows Rava. I suggest the following explanation. There is a rule that the Halachah follows the later opinion from the time of Abaye and Rava and onwards. See Tosfos Kidushin 45b DH Haveh that this is because the later generations were more accurate in fixing the Halachah than the earlier ones.(See also Rashi Nidah 7b DH ha'KM"L). See also Tosfos Berachos 62a that Rava only became the king (i.e. the Rosh Yeshivah) after Abaye died. Since Rava was after Abaye, this can explain why the Halachah usually follows Rava, because in the later generations, the Halachah follows the later opinion, as we just observed.

The questioner in Teshuvas Rivash, end of #405, suggested that the reason the Halachah follows Abaye in Lechi ha'Omed me'Eilav (one of the Halachas cited by Rashi Bava Metzia 22b) is because there is a rule that the Halachah follows the lenient opinion on Eruvin (see Eruvin 46a). Rivash rejects this argument because the Halachah follows Abaye in Gitin 34a even though Abaye is lenient there and Gitin questions are generally severe because they involve permitting a married woman to remarry and there certainly is not a rule that one always follows the lenient opinion on Gitin. Therefore Rivash states that in the six cases of Ya'al Kegam, the Chachomim knew that Abaye's opinion was the correct one and therefore they ruled according to Abaye. In our Gemara in Bava Metzia this is easy to understand because the Gemara says that Rabbi Yochanan's statement in the name of Rabbi Yishmoel ben Yehotsadak disproves Rava. In Kidushin 52a, the Gemara also cites a Beraisa which disproves Rava. In the other four places, Chazal must also have studied the subject in depth and decided that Abaye's arguments were more convincing, whereas in all their other disputes in Shas, Rava's reasoning had the merit to reach the correct opinion.

Kol Tuv,

D. Bloom