This rivalry is personal. It's common to find CU students on the CSU campus at a party or vice versa. It gets ugly due to seeing someone you don't like 365 days of the year. CSU (& Utah) never forgave CU for leaving the Rocky Mountain Conference for the Big 7 (Big 8/Big 12). CSU wishes the worst on CU for that move. CU has never looked back. Also, if CU offers a local kid, then CSU offers, chances are the offer may not be committable to CU.
I remember playing for Coach Mac, and the CU/CSU game was the only game where any personal fouls didn't carry any punishment the following week. Coach Mac was religious, and the CSU game was the closest I've seen him curse. As you can see, I don't like CSU anymore than I like State Penn or Sorrycuse.