Zoomies quit the series because they were regularly getting their asses handed to them and physically getting beaten up every game they played CU. Anybody says anything else, they're lyin'. Crowder's teams played a very physical brand of ball, you might beat the Buffs, but you lost half your starters for the next game. The teams who followed CU on other teams' schedules would usually win, since the opposition was so beat up after facing CU.
The real nail in the Zooms' coffin, was the freshman game in '69 (Yes, they had a frosh squad back then, coached by Dan Stavely). LB Jeff Geiser (later Dr Geiser, PhD) personally destroyed a couple of AFA players with vicious, but clean and legal hits, I think breaking one kid's leg. You could really hear that hit and know something real bad happened to the AFA RB.