When you hire new coach, he needs to be supported by admin, fans, boosters, etc.
"New Mexico’s attendance [vs Wazzu in win] was 14,067, giving UNM an average attendance of 16,001 for the season. That’s the highest average attendance in a non-New Mexico State home game season since 2016 when UNM averaged 20,277 which included the New Mexico Bowl."
I suspect Bronco didn't feel like he was getting or would ever get the support necessary to be successful there. This was probably very disappointing to him given his history there. I believe he would have stayed if he felt supported, as he was at BYU and UVA.