The simplest answer is probably the right one: Gemini, or at least the Gemini models that Google believes it can serve to customers in a cost effective way, simply isn't up to par. A release where Gemini manages to be at where OpenAI was three months ago would objectively be an achievement, but it'd be a disappointment and disastrous for investor's outlook for Google. So the delay is to kick the can down the road a bit and hope for some miraculous development that substantially improves quality.