As much as I hated the fact that Cassie got the most solo missions, I think they all sort of did in some different ways. I mean, maybe Jake and Marco got left out for whatever reason, but Ax was pretty much on his own for the entirety of #38 (at least we thought he was), and there was that same "And Then There Were None" thing going on in #18 that was happening in #29. Tobias got a hell of a lot of solo missions--#3 was all about him and his angst, most of #33 was on his own, etc. And Rachel got a couple as well--as much as #22 was a team effort, she basically trapped David on her own (all right Ax was there but he didn't do much), and so much of that book was internal that I don't think anyone defines it as a group mission. And then, #48...or #49...idk at that point in the series I lose track, but that whole David-Rachel-Crayak showdown can be defined as nothing except "solo mission."
But yeah, why were Jake and Marco left out? Maybe because Jake's role/character development was invalid unless he had people around to lead, and Marco just works better if he's playing off other people? Idk, I'm probably forgetting about a couple of books