I've been thinking about it, and it may well be that they don't have the technology at present to give us Makeb in the CM.
Oh, they definitely have the ability to restrict zones, as evidenced by Section X being a cartel market purchase.
But, as with everything else about this content update, the level cap increase ruins everything. The existing mechanic to impose a different level cap came with the trial, and that was all account-wide and all done out-of-game.
Using an ingame item to raise the level cap may well be trickier for them to code. I have little doubt they'll add it eventually though (although whether or not they do so only after it's become obsolete, like WoW charging five dollars for outdated expansions, remains to be seen).
Well they seem to have the technology to sell Ch4 on the Cashshop, which is how it would be done I suspect, Makeb in chunks, and is possible how F2Per will have to buy it.
Certainly I can think of no great technical reason, and any that may exist (like level cap) will HAVE to be resolved before Makeb goes to early access anyway.