My extensive research led to finding there are no laws prohibiting the hiring of convicted felons.

My question is:

What are the reasons employers will not hire any that is a convicted felons?

Im not sure where your looking but there are some jobs that you can not hire a felon for. One such example would be that you can't hire a felon to run the counter in a firearms store. Another is in the trucking industry there are areas where you can't hire a felon. You need to look a little deeper before we can begin to answer your question.

Libraries do not hire them because money is regularly handled by the staff and relationships with the public is very important, so a high standard of employees has to be met.

Simple, there is a saying that a leopard cannot change it's spots. Most employers will fear that a convicted felon will steal again.

I would think that's obvious.

... Most employers will fear that a convicted felon will steal again. ...

Or, if not steal, commit some other crime.