It's because most people don't do day trips to Bryce Canyon or Monument Valley from Las Vegas. Monument Valley is simply too far, and most visitors will stay there or in nearby towns (like Kayenta as cblaisd mentioned) so guided tours typically start at Monument Valley. Even Bryce Canyon is ~4hrs away, which makes a sunrise arrival a ridiculous proposition and with any reasonable departure time from Las Vegas, makes for a tough schedule if even the most basic hiking like Queens/Navajo is desired in addition to visiting the typical panoramic points. Bryce Canyon can definitely be done in a day, but that assumes an overnight at some point - not driving for 8 hours.

I'd probably avoid bundu bashers - they have a plethora of bad reviews on the Internet, F rating with the BBB, etc.