That’s something I wondered about recently. In ANH, the fact that officers are so dismissive of Vader and his bygone religion implies that most Imperials had very little regard for the Force or its users. The whole culture of the Empire is very mechanized and anti-spiritual, which isn’t what you’d naturally expect from a faction led by a Force user, even an evil one.
I think the decision to make him a Force-using dark lord came during the writing of Empire. I think it was because they wanted to have a big bad that Vader would kneel to, and the only kind of character where it would make sense for him to do that would be another, more powerful Force user.