Perhaps, they need more clarity on the whole system and its processes or waiting for more opportune times. It is often said that big investors know something that the rest haven't known yet, that could also be a reason why they haven't entered yet.
Clarity, ok, I can see why they can't plunge into investments they might consider risky for various reasons since they need to follow certain rules. A better timing? I hope not! A better timing than one week after ETFs were approved would mean Bitcoin gets back to 30k, and I don't think it's possible soon, and maybe never. A better timing than now? Yes, that's possible.