Dude MLS is in FIFA yeah but nobody watches MLS. There's no point in watching MLS, you can find Serie A, La Liga, EPL and Champions League so easily. Even the French league is badass.
The NBA is the single most popular sports organisation in the world. I'll see if I can dig up some figures.
I said more people have at least heard of the MLS.
And some people just watch all they can. It's like saying why watch Bellator if theres UFC you know.