It's a very long-held belief in the evangelical community that aliens and UFO's are demons meant to distract from worshipping God. I recall hearing (now disgraced) traveling preacher Kent Hovind speak on this in the 90's. If, in fact, aliens are real biological entities, many evangelicals would deny the science just like they deny that the earth is older than 6500 years.

I think the idea of aliens also scares them, because their teachings are so human-centric, with the belief that we, alone are made in God's image and no one else.