Different reasons for different states.
Many times there is a selected state that they want to put the candidate over the top. It helps secure the base for that state. So they will simply do that math and realize that somebody needs to pass so that the candidate does not get the needed votes (I think 1255) to be put over the top.
Others want to pass so they can do it at a time when they think they may get more exposure....and help the GOP in that state grow.
And there have been times when a key person for a particular state...that they want present...has been delayed. That is a rare case.
Neil Bush & kids are on c-span