1. The assets for basketball (uniforms, court, hoops, etc) are MUCH more easily available (often for free) for Daz3D.
2. Did you ever hear of MARCH MADNESS? Michael Jordan, Shaq, Kareem Abdul Jabber, Dennis Rodman and many others transitioned from basketball fame to films (although the quality of those films varies wildly.) Basketball has consistently been in the Summer Olympics since 1936 (originally introduced to the Olympics in 1904!)
3. Many AVN devs (and the writers/artists/etc that work with/for them) are not located in the USA. They're probably not familiar with American Football. They probably don't know how popular "Soccer" (aka. Football anywhere else in the world) is here nor how popular Lacrosse (not just for stoned Ivy League colleges anymore) is, etc. They've probably never even heard of field hockey. They probably overestimate the popularity of baseball in America (it ain't the 1940s anymore) and probably don't even know the basic rules, positions, etc. Basketball is an American sport, easy to understand, and which is known/popular around the world.
4. Seriously, have you never driven around your neighborhood and counted how many houses have basketball nets in their driveway? Basketball is HUGE and available to almost anyone - you just need a hoop and a ball.