There's no proof that it is like that. Last official talk between MTA devs ( back then Kye was mta dev too ) and R* was back in 2003.
After the hot coffee drama rockstar no longer supports modders at all.
What made mta like it is is the open source license, therefore much more commiters, much better organized work with all the bug trackers etc, and clientside scripts that pretty much is the cause of all the advanced features.
Why sa-mp is more popular? Its simple.
1) while MTASA:race came out first, it was too limited and lacked any scripting, then sa-mp released as a big breakthrough.
2) google "gta sa multiplayer", bet what name is more obvious and shows first?