Muman that is a great question (I had it recently) about the Bat Kol and why in this case it decided (seemingly) the Halacha yett with the story of the "oven like the snake" where it did not.
Here is the answer.
http://www.dafyomi.co.il/bmetzia/insites/bm-dt-059.htm 59b----------------------------------------59b
2) TORAH IS NOT IN HEAVEN
QUESTION: The Gemara relates the incident of "Tanuro Shel Achna'i" and the miraculous ways in which Rebbi Eliezer attempted to convince the Chachamim that the Halachah was in accordance with his opinion. He miraculously uprooted a carob tree, had the waters of a river shift direction, and caused the walls of the study wall to lean inwards, and still the Chachamim did not change their mind. He then declared that Heaven should prove that he is correct, and a Bas Kol emanated and pronounced, "Why are you challenging Rebbi Eliezer, when the Halachah is like him?" Rebbi Yehoshua stood up and declared, "Lo ba'Shamayim Hi" -- "It (the Torah) is not in the heavens!" (Devarim 30:12).
TOSFOS (DH Lo ba'Shamayim) asks that the Chachamim had a different reaction to the Bas Kol which decided the question of whether the Halachah should follow the view of Beis Shamai or the view of Beis Hillel. The Gemara in Yevamos (14a) relates that when the Bas Kol declared that the Halachah should follow Beis Hillel, the Halachah indeed was established in accordance with the view of Beis Hillel. Why does the Gemara here say that the Halachah is not decided based on a Bas Kol?
The MAHARAM explains that Tosfos is not asking his question on the view of Rebbi Yehoshua, as the Gemara in Yevamos itself cites Rebbi Yehoshua who said there as well that we do not rely on a Bas Kol. The question of Tosfos is on the Gemara in Yevamos (and other places) which follows the view of Beis Hillel because of the Bas Kol. If the Gemara there follows the view of Beis Hillel because of the Bas Kol, why does the Gemara here not follow the view of Rebbi Eliezer because of the Bas Kol?
ANSWERS:
(a) TOSFOS answers that in this case, the Bas Kol contradicted the Torah's principle of "Acharei Rabim l'Hatos" -- "turn after the majority" (Shemos 23:2). In the case in Yevamos, the Halachah presumably followed the view of Beis Hillel, whose opinion was that of the majority. However, the fact that the students of Beis Shamai were sharper caused a doubt to arise about whom the Halachah should follow. The Bas Kol resolved the doubt when it proclaimed that even in this case the verse of "Acharei Rabim l'Hatos" applies.
(b) In another answer, Tosfos explains that the Bas Kol in the case in Yevamos was spontaneous, and therefore considered more "neutral." In contrast, the Bas Kol in the case of "Tanuro Shel Achna'i" was in response to Rebbi Eliezer's demand that "Min ha'Shamayim Yochichu," and therefore the Halachah does not follow its declaration. (Y. Montrose)