Takes time to get an initial review of the play to see if it needs an replay review. Keep in mind it's not like this takes minutes - the play clock is only measured in seconds. It has to happen pretty fast.
It seems that everyone else knows there needs to be a review before the refs call for it. Teams try to rush to get a play started. I wish they would call for itore quickly if they are going to do so. Maybe they have some system of confirming the need for the review. If it is just one guy making the call for the review, why does he so often take so long?