The problem is that CN/WB shows for kids don't last long enough to be incorporated into the parks as permanent additions. Examples; Adventure Time and Regular Show is huge on CN, but because probably in the next 2 or 3 years those shows will get cancelled or run their course, there would be no use for the characters in the parks. But something like Looney Tunes and such have been cemented as American Icons, so regardless of their relevance to the kids, their image will still be friendly and inviting to the younger crowds.