I'm not sure it's true that they're never rotated. The 738s I've been on that CMI was operating were owned by Continental Airlines and with the exception of the plastic emergency exit signage, they were identical.
Additionally, CMI operates COA B764s -- on flights 1 & 2, the same airplane flies the GUM leg as the IAH leg, but under the name of the different airline.
Well never is a very long time. But it's true that the 737/8's aren't rotated on a regular basis. Recently Some of them have ben upgraded with ADS/CPDLC. I assume that was done off-island, so they would have had to rotate the planes though hawaii on the "island hopper".
I thought there were at least 2 Air Mike 764's. The Air Mike 764's have a much smaller business first cabin then the rest of the fleet. There is also a 764 used between GUM and NRT twice daily. I believe those also have the smaller F class.