I spend a lot of time modeling and tracking viewership and I'd be shocked if Utah/Arizona draws anywhere near as many as BYU/K-State. Part of that is because 1.5M is a great number for the spot and so the best guess is the under but there's 3 reasons that I'm sure of it, even before we get into any difference in BYU or Utah's drawing power

1. MATCHUP: 3-0 BYU vs 3-0 Kansas St were both undefeated. A matchup of unbeatens is going to inherently have more interest than 4-0 Utah vs 2-1 Arizona.
2. OPPONENT: Having tracked viewership for the last 7 years, Arizona tends to draw terrible numbers.  Kansas St is a much stronger draw (maybe the fanbase, maybe the location, can't say why).  Even if the two teams are of relatively even quality, Arizona has been substantially worse at getting viewers.
3. COMPETITION: This is probably the biggest reason.  BYU/K-State was the only game going on at that time.  Utah/Arizona has to go up against Wazzu vs #24 Boise St (on FS1, so not a huge issue) and also #8 Oregon @ UCLA (on FOX). That means Utah will be the #2 game for most people instead of the BYU game, which was the default top option

Other factors which could play a role (but don't really this time) are the quality of the lead-in game on the network.  BYU had 7 Miami @ USF.  Utah gets Stanford @ 17 Clemson.  That's pretty close to a wash.