If Derek could handle the "psycho Dad" and make him change his mind or treat Maya better, don't you think he would have done so already? Derek seems like he's pretty feckless when it comes to this stuff and really isn't much help at all. You even see it in Ep 3 where, after their Dad leaves, Derek leaves too and doesn't stay to talk with Maya or console her in any way. He seems to have an attitude of, "Well, that's enough heavy shit for me. Goodnight, sis, I'll see ya tomorrow." The MC at least talks to Maya about what's going on with her and really tries to comfort her as seen in Ep 2 after the cum-petition, Ep 2 if he leaves the party to go back to her, Ep 3 when he talks to her before going for that mini-date on the picnic blanket, and Ep 5 if he goes to see her instead of Josy.
He's clearly much better at offering support and listening to Maya than Derek seems to be, and that coupled with the fact that they both have feelings for him even on the non-M&J makes him the obvious choice for someone they feel they can rely on.