I'm not sure why it hasn't happened yet. But Marvel should get more proactive in adapting vintage storylines from the 1960s until now for straight to DVD movies. No need to be hung up on continuity from one project to the next. DC/Warner has been doing this for some time.
Lots of stories could be adapted, not just high profile maxi-series like Secret Wars or Secret Invasion. Also storylines that were internal to a particular series like Kree vs Skrull War or the Korvac saga from The Avengers. If Marvel wants to be mindful of its MCU-related branding, they can choose to include storylines from characters like Captain America, Thor, Hulk, etc.
In particular I'd like to see the early Walt Simonson Thor arcs adapted into an animated feature.