I think LP just sells a lot more than Muse and brings bigger crowds in the U.S. because they've been a big staple here for such a long time. All the singles off of HT and Meteora are huge, there's WID, SOTD, WFTE, and now BID to keep everyone's interest and what not. People here are just now getting used to Muse being this huge act. The only songs most people know by them are Knights of Cydonia, Uprising, and Madness.
To be honest, I think that LP and Muse are RIGHT at the same place, fame-wise. They can't both headline the same show, nor can one open for the other. So one of them opening for Metallica had to be the compromise, since Metallica opens for no one, lol.