My understanding is that at least in modern times, it's because there are some rights issues – I think Universal has (or had) distribution rights to Hulk movies. That's why, after the Edward Norton movie, Hulk has only been around as a guest star in the Avengers movies as well as Thor Ragnarok. If they make a "straight" Hulk movie Universal gets a piece of the pie, and why do that when you can instead do a movie where Marvel/Disney gets all the monies? But that means that there's no room for Hulk's supporting cast, given that Hulk is relegated to a supporting cast role himself.