I think there's a perception that the GP's aren't for beginners.
Having an Open class will do little to change that perception, SFT grade was possibly more successful in that respect.
It's not hard to see why the GP's attract that perception. First off 50 targets against the clock, courses tend to be very long - some more than others. The name doesn't do much for bringing in newcomers - when you're a fresh faced FT shooter, are you going to have the balls to enter a National Grand Prix competition series?
Personally I don't think it's a problem though - the Regional Winter League shoots are there to bring newcomers into the sport and feed into the GP's over the summer.
That probably explains the lack of C grade support for the GP's too.
My perception of the Open class is that it was introduced in an attempt to try to solve the Dispensation Rules problem the BFTA painted itself into a corner with. I'd imagine I'm not alone in that perception.