I think Booker, Ayton, and Rubio sat out for a few reasons mentioned here already. I think solidifying who would be the backups was something Monty was able to do easier without them taking up minutes. Their roles are known and that isn't changing. He could rely more on Carter, Jerome, Baynes, and Kaminsky tonight and also get a better look at Tyler Johnson, Saric, Bridges, and Oubre.
I think Bridges and Oubre are also competing for a job and moving Booker to the bench for the night allowed them both to start together to see what they can do as starters. Since those 2 missed a lot of camp, or didn't participate in a lot of camp, they needed to work them back into things. Doing so without the others could have been easier for everyone because they're coming in to be the 3rd or 4th option but without Ayton and Booker, they can be the #1 option and focus a little more on getting themselves into the flow of things rather than getting the other guys going.
It wouldn't surprise me if there was also a bit of talk about the Blazers having the Suns number. I know they've turned over a lot of the roster and the coaching staff but that doesn't change how the Blazers have handled the Suns the last few years with numerous coaches and players involved. Taking our big 3 out of the game allowed the Suns an excuse if they lost but it also helped show that they can win and it's a new team and culture here. No one should have our number because it was changed this offseason.