Often it's because one goes against the fundamental beliefs of the other.
There is one here on HubPages who refuses to believe that there is anything at all in the paranormal. Nothing you can say to him will make him change his mind either.
I think it's because for the scientist, there is a need for empirical evidence and that's rarely possible with things like the paranormal or spirituality.
The same can be said in reverse for spiritualists who don't need anything empirical, but are prepared to believe something intangible. However, it is possible to get a spiritualist to believe that scientific things are not always the work of a higher power.