I suspect that it was because the Doom Patrol had a definite ending.... you couldn't get more definite than killing off the whole team!

I always felt that the problem with Atom and Hawkman was the fact that their solo titles merged into one comic, and several of these stories were full-length team-ups, meaning there would be a certain amount of duplication between the two if they were reprinted as archives. Particularly with Hawkman, as he also had some crossover stories printed in the Adam Strange Archives. Theoritically, both Atom and Hawkman could go a few extra volumes because after their solo and joint titles were cancelled, they lived on in back up strips for years. Adam Strange could also go another volume, collecting the later stories illustrated by Lee Elias, a couple of original stories in late issues of Strange Adventures, and his back-up strips in Worlds Finest and Green Lantern.