The reason we have a trinity system is because it's a tried-and-true formula that's easy to design and balance around, and that any combat system with any real level of depth must support a system with at least as many roles. One pillar of the trinity controls threats, one pillar supports the party, one pillar murders threats. There's a little wiggle room for hybrids, but building a hybrid such that it can handle more than two roles at once is a balancing nightmare. A little too weak, and it's useless. A little too strong, and it's overpowered. AST, with its party buffs, already treads that awkward line between under- and overpowered. And if you do get that magic balance right, you have to keep designing around it and tiptoeing so as not to upset it. Hybrids centralize encounter design and job balance around themselves like gravity wells.

It's much easier, much less of a headache, to design more straightforward jobs with interesting gimmicks than to develop true hybrid jobs.