Jimmermania does in fact have something to do with the way fans feel about Rose. That's pretty straightforward. Fans get emotionally connected to plays, events, eras, etc. Jimmermania is the biggest high surrounding either football or basketball since at least 1996--maybe 1984.
The difference between Rose and Bronco isn't that extreme. To that extent, you're right--people make the gap seem huge, when it's not. But those that can't see the differences that are there, IMO, are willfully ignorant.
Here's a rough breakdown of the few differences:
Under Rose, BYU has had a team that was considered, even late into the season, a true national title contender (see BYU's #1 seed projection before the Davies incident). He's had what most die-hard basketball fans consider the equivalent of an NY6 game. He's had the equivalent of a Heisman winner. Recruiting is at an all-time high. The feel around the basketball program is just plain better than it is around the football program. People feel that if Rose was able to assemble a potential national title contender in 2011, with all the incoming talent he has lined up, there's a good chance he may be able to produce a Top 5-10 team again.
Bronco. BYU has been solid, but not spectacular for quite a while now. 8-5, contrary to CB's opinion, is not bad--it's pretty good. About like making the NIT, as most seem to see it. Nothing to get too excited about, but nothing that should have anyone saying we were horrible. Early in his tenure, he did have several Top 25 teams, and that's a mark in his favor, but that's been 6 years ago now--which is the longest stretch that BYU has not had an AP ranked finish since Lavell first landed BYU in the Top 25 at seasons end back in 1977. BYU has not had any national title contenders in football under Bronco. No major bowl--no top 10 team at the end of the regular season or the postseason (after the 2009 Vegas Bowl, we got close, with a #12 finish) No Heisman--and no major awards. Recruiting is solid, but not great.
Those few but key differences are the reasons that many feel differently about the tenure of Rose compared to Bronco.