As far as I'm aware this very rarely happens so there's not enough examples to find a pattern and guess at why ZOS do it.

But I assume it's the same sort of reasons any company would delay or cancel a piece of software they were planning to sell, most likely they find some kind of problem with it which means it won't work properly and releasing it as-is would annoy or disappoint people or cause further problems. For example they delayed all the multi-rider mounts for a while because they found it triggered their anti-botting code and people got automatically banned for using them.

It's probably a different problem for each item, unique to that one, rather than some sort of wider trend otherwise we'd see it happening more often.