I think it's because her music/lyrics are perceived as being very cheesy/dated/generic... which, they kind of are tbh, but she's still a guilty pleasure of mine when I'm in a certain mood.
Love her All the Way duet with Frank Sinatra. And her last album.. loved Breakaway and Water and a Flame.
Not this thread being made just as i'm listening to I Drove All Night. She's even underrated as a vocalist here imo.
Celine is apart of the vocal trinity for a reason.