Too much competition from other big summer tours. There's smegging loads of them in America now- Mayhem, Uproar, Honda Civic, Carnival of Madness, Trespass- all of which have only really came up in the last few years.
The fact that LP aren't as big a presence in the media now doesn't help, very few bands can prop up festival tours all by themselves now anyway. Metallica are pretty much the biggest band in the world, and they only do one off events with Orion.