I just reckon it's the simplest, most obvious reason:
It's turning out to be a lot more work, with a bunch of inherent difficulties they didn't expect. And quite possibly, they're doing the opposite of what happened with GTAO's launch, in response to how all that panned out - I'm assuming they figured the complaints they'd get for a delayed release of heists would be the lesser evil, compared to the inevitable sh*tstorm that would happen if heists launched as a broken, buggy mess.
If that's the case, fair enough - I'd rather wait for heists and them be playable, rather than have them sooner, broken, and effectively delayed until all the bugs can be ironed out. Better iron 'em out behind closed doors and just give us the polished product, says I.