I'm not sure about anyone else, but maybe its because of their uniform or something or maybe their involvement in the chapters of the book (talking about chapters compiled to volume 20). I mean, first theres the fact that only Allen and Kanda are wearing those types of exorcist uniforms (yeah, just talking about the exorcists. 3rds not included. So is Link). We can excuse that though, cause they were both assigned in desert locations and their original uniforms would've been too hot to wear.
The involvement part, I still haven't placed a theory on since a lot of people say it should have been Kanda and Alma instead of Allen in the cover. Well, maybe its because for one, their the only exorcists focused on all those chapters (well, mostly Kanda and Alma which is why people are complaining so much), and for the other, because they are both reasons for the invasion of the North American branch. That bring two major points.
1. That Kanda is needed to awaken Alma
- which is why he gets cover page, heck, he's had most of the spotlight.
2. That the Earl (and maybe other Noahs) want Allen to leave the order
- which still doesn't explain why Allen gets cover page in 69 position with Kanda. XD I actually love the cover but I wont fangirl over it, CAUSE THATS JUST ME.
(*cough*cough*)
NOW. On to the theory of why Allen and not Alma gets cover page.
It started at the end of vol.19. I mean, the Earl did bring Allen all the way to the NA Branch for a reason more than being an audience to Alma's awakening. I cant really explain it, but even though Allen was barely focused on the chapters, his presence in the branch was obviously important. How he reacts to the events and his choices in future chapters, Kanda's memories trigger them. The Noahs are trying to get Allen to leave the order, right?
Of course, like I said earlier, this is all just my theory. Its not yet done, but long explanations(especially those I cant quite put in words) are not my forte, so I'll just leave it here.