Perhaps they thought it incidental, and perhaps they were trying to fly under the radar of the Archdiocese of New York.
Let's remember how long ago this was....and how many things you could not do, and could not say, and could not show on television.
I have no doubt this show in particular was under constant scrutiny to be sure they were not crossing that invisible line between what you could get away with, and what would generate a call from the Network. After all, the whole premise of it was the Supernatural and that was shaky ground.
It was obviously okay to hint at the DuPre's Catholicism...after all, they were French. But they had to be very very careful that they didn't emphasize it too much, or offense might have been taken, causing the production no end of trouble.
I suspect that's the real reason the exorcism was so lame and bore no resemblance to the real thing. THAT might have prompted a call from Cardinal Spellman.
Raineypark