It's probably not as common as it has the potential to be because A) People still associate it with Bewitched and witches in general, and they don't want that image associated with their daughter, and B) The sounds don't quite fit in with what's popular at the moment.
I don't think it's religious-sounding at all. I wouldn't associate it with anything related to any religion.
I love Tabitha and would definitely consider it for my own child.