Just a thought, but when we were a Premiership team (and I know it's not the same) and getting crowds of 20k, were we all arseholes for not filling Cov Rugby as well?
Seems to me that for the vast majority of people, they just want some entertainment. Lower league stuff doesn't tend to provide that (as we can all attest over the last few seasons) so they go for the higher stuff.
There's something like 305k people in Cov, only 5k of them are City fans. I don't even like rugby, but I've been invited to a game so I'll go for the social occassion and to see what it's all about. Much like many of those in the crowds at HR when we were in the Prem. We didn't attract 23k when Man Utd came to town because of the loyalty of Coventry people.
There's a little bit of "Old man shouts at clouds" about this thread and the general sentiment of this site. It's not some great mystery why a team higher up the leagues gets bigger crowds.