It seems completely obvious in retrospect, but it took a random interview with Mark Ruffalo to explain the likely reason why there's been no solo Hulk movie yet — it's Universal's fault.
Now, Universal doesn't own the rights to the character of the Incredible Hulk, obviously; Marvel does, which is why he can appear in Avengers movies. But thanks to their partnership with Marvel on 2003's Hulk and 2008's Incredible Hulk movies, it appears Universal still has to be somewhat involved in all the Hulk-specific movies Marvel makes — which means they'd get a cut of the profits, too. Disney can't be too fond of that, and it makes spend that they'd rather spend their resources making Marvel movies where they collect all the earnings instead of dealing with the green goliath.
The good news is that whatever Marvel's deal is with Universal, it's not like they sold Hulk like they sold Spider-Man to Sony or the X-Men to Fox. They basically just promised to cut Universal in when making Hulk movies. We don't know how long this particular contract lasts, how desperately Universal wants to keep this piece of the Marvel pie, or how willing Disney might be to negotiate. As Ruffalo said in his interview with Collider:
As far as a Hulk movie, a standalone Hulk movie, Marvel doesn’t really have the rights to that yet. That’s still Universal’s property, so there’s that issue. That’s a big impediment to moving forward with that. Now I don’t think that’s insurmountable, by the way, but I don’t know where it’s going from here for me.
But at least now we know!