Well, apparently it's to do with the fact that they're welsh. UEFA has said that their only route into Europe would be via the Welsh Cup (which they can win and then get into Europe, IF they get through those endless preliminary round things in the summer) or the League of Wales if they took part.
Presumably this also means that if Cardiff got to the Prem and got to say 4th place, they still wouldn't make the CL.
I agree, though, it's an interesting question and I hope it happens. I like these little football irregularities. Enjoyed all the conundrum over Liverpool winning the CL and not making 4th place a couple of years ago, although I was kind of pissed off that UEFA granted them special dispensation. Liverwank do get away with a lot I find.