None of us knows why this shidduch is suddenly unappealing. Nevertheless, unless there is a very real concern, i.e. you discovered he is a liar/phony or a gambler, or abusive, etc. then you would be wise to keep your feelings to yourself and make every effort to re-find what it was that made you think he was right for your daughter in the first place.
Whatever it is, here’s a lesson for us all. Listen to ALL suggestions that people have for shidduchim and don’t discard them out of hand for foolish reasons. Maybe they are “better prospects,” maybe not. But it sounds like you feel you rejected them for the wrong reasons and now have charata. The lesson learned is that we don’t usually get a do over. So we need to try to get it right the first time around, and really give proper thought when making a decision as serious as accepting a shidduch.