The Babylonian Talmud does not have Gemarah for the majority of the tractates in Zer'aim, the first of the six Mishnah "orders". As a matter of fact, only tractate Brachot has Gemarah. There are other tractates as well, such as Avot, that have no Gemarah. I don't recall the full list.
Regarding why Brachot is the only tractate in Zera'im having Genarah, I read (don't recall the source, offhand. B"N. I'll edit, later.) the reason is that other than Brachot, the other tractates deal with laws applicable only in the land of Israel and have to do specifically with land laws. Thus, it was not applicable in Babylonia. (As CashCow mentioned in a comment, the Jerusalem version has Gemarrah for it.)
I can't explain why each of the other tractates have no Gemarah. IMO, Avot, could have used one, but, then, if it did, we might have put much of the current marketing of Avot commentaries out of business :-)