I think your question has more than one answer. Of course I'm giving opinion rather than solid evidence but:
First off some like to seem to be "in the know", but chances are they are no more "in the know" than the rest of us. For sure some may be in positions where they ought to be able to make a better guess than the average individual, but being human they will also have personal preferences which may colour the interpretation they use, the assumptions they make, and any conclusions they draw.
There again some might have a need to 'stay in' with the powerful in the hope of benefiting from the gravy train, golden goodbyes, whatever as a personal gain thing. Common sense tells us that this is going to be the case regardless which individuals one suspects and regardless of available evidence until it's too late.
There again some may have a job where it will be easier for them to get results if they don't have to deal with separate bodies/countries/whatever and can instead just go along with the large group. The idea of having to show some skills and really achieve for a change, would be a little frightening. They may show themselves not to be one of those folk you must give whatever salary they demand or else they go elsewhere, after all.
Still others may be pessimistic with regards to change. It is human nature to want to stick with the familiar. The devil you know. They would be adverse to any small amount of turbulence as things settle down. The fact that it may be, and IMO almost certainly will be, better for the UK after voting for change, does not convince all.
There may be more reasons but perhaps that will suffice for now.