If you listen to what he says, and the emphasis that he puts on having an impact, I think a lot of what is said above is correct, but we are missing the connection between a few key dots.
CU is a ready made opportunity for him. World class facilities. World class town. P5 conference at a school that he can start from scratch and build it his way, at a school that has given multiple AA coaches an opportunity. A school, with history and tradition, that has won it all in the past. I think these attributes are all obvious, but I think there is potentially a deeper connection beyond this, and I point to his relationship with Kordell Stewart to provide a window into that deeper connection.
Kordell was talking with Prime about how he told him this was God's country. I would bet a large sum that Kordell talked to HCP about how Mac was a man of faith, and how that leadership fit into his experience in Boulder. Prime is clearly a man of deep faith, and he utilizes that faith in every decision and path he takes. When he was talking to Kordell about this, it seemed to hold a deep powerful meaning to him. Why? Because of his mission.
His stated mission in just about every decision he has made over his coaching career seems to be grounded in leading "his people". By his people, I infer people of faith, people from a disadvantaged background that LOVE football, who are looking to use football to better their life. A large portion of the kids/men that fit this description also happen to be largely represented by African Americans. So, why Boulder? Beside all of the ready made, turn key reasons above, Boulder IS lily white and extremely wealthy. If you are a man on a mission to lead "his people", is there not a better environment to help your people grow, mature, and advance, then a safe, rich, educated town that likely represents the most drastically opposite environment from what many of these kids have grown up with?
"Son, come to Boulder and I KNOW it will prepare you for any challenge you will face for the rest of your life. You'll play for championships against the best in the country and we will show you how to become the best FB player you can be. But here's what else you will get from the University of Colorado; a world class education. You will be in one of the safest towns in America, and in this environment, you will largely be surrounded by well educated and wealthy white people. Whether you play in the NFL, or you go on to be an aerospace engineer, or a business leader, who is it that you think you'll largely be interacting with after you leave college? This will be a full life growing opportunity. athletically, spiritually, intellectually & socially. You will leave CU as a young man ready to take on any challenge."
For a man that loves to take on challenges, wants to help people grow while he competes for it all, wants to do it his way, and wants to lead young men in all aspects of life....is there a better place or opportunity than this? This is what I think Kordell and Prime discussed, and IMO, HCP confirmed in his public discussion with Kordell. Separately, I have heard Kordell talk about this very thing (particularly learning to interact with, and be comfortable with white people while he was in Boulder), and I bet this whole life picture and growth opportunity is "Why Boulder"