I figured Utah drivers were just bad at this. The last time I went to California I decided to watch closer. California and especially Las Vegas drivers were so much worse at staying in the left lane than I've ever experienced in Utah. The absolute worst area was I-15 between Vegas and California both directions. Every car would just stay in the left lane, even when there wasn't much traffic.
And no, it wasn't Utah drivers coming back from Disneyland. They were Nevada and California plates. My wife and I were laughing at how bad it was and noticing the plates as well.