Because she’s a sex positive person and enjoys having sex, of course!
Or because I’m infatuated with her and I like drawing her enjoying lots of sex!

But I’m not sure where that “having sex with so many ponies” comes from, actually! I’m not really drawing AJ having sex with so many different ponies. And in my head she’s also not constantly in a monogamous relationship with Rainbow Dash, while it’s true that I ship her with RD, I also ship her with Rarity and I can see a lot of other ships involving her being cute or hot and sometimes I even really like the idea of the characters being poly.

I just draw what I’m in the mood for and what I think is fun, my drawings aren’t supposed to be all part of one big interconnected overall story or universe. 

Sometimes I just like drawing AJ (or any other pony) being sexy or having a good fuck and I make up a small scene/background story for it in my head, but it’s almost never related to any of my other drawings.

You could say all of my drawings are separate little what-if scenarios.