SA:MP is more popular because of 1 simple reason; they were first. (2005?)
They had a good start, and then many years later (1 jan 2008) MTA came with the deathmatch version (DP1) and it was still buggy after all those years of development.
Now many years later MTA passed SA:MP on every front. Not all players would just move over after so many years, they got used to SA:MP and are very attached to the community.
Atleast that's what i think.
Though i am very impressed how popular MTA is today, didn't expect that at all to be honest.