I have the opposite question - why Eros, bestgfe or Tnaboard and not STG?
At least in the AC area, STG seems to be the only site the girls actually post to on the regular. Eros is all suspiciously airbrushed photos of very similar looking ladies with unfamiliar area codes (ie: scam alert), and for the life of me I've never been able to figure out the other two sites.
Yeah, being the most popular option means ALL the providers are there, meaning a lot of junkies and flakes. But it also means most of the decent options that won't break the bank are there too. You just have to be smart, read reviews, look for the red flags, etc.
Also, shout out to Listcrawler and Tryst - they're runners up in terms of real girls actually posting to them. Listcrawler being the super ghetto option (STG on... let's say methadone) and Tryst the "upscale" one.