I've been told that Niddah is the only masechet in Taharot with Gemara because it has practical applications even when there is no temple.
But the halachot of Mikvaot are just as important nowadays as those of a Niddah. Why didn't the Amoraim make a Gemara for it?
Edit: I believe my question is not a duplicate. It acknowledges that there are reasons that most of Taharot don't have Gemara, but questions why specifically Mikvaot wasn't an exception like Niddah was.