A couple of things here and I will address each individually.
Johnson is correct about why D-I Football coaches are not permitted at the clinic. Supposedly, that will change after this year with the FB and BB all-star games moving to the end of respective seasons. (At least that was the plan discussed at the end of the clinic last year..) Note to that: BB All-Star games are played in a gym (LC) that isn't as nice as many HS gyms I've worked in rather than playing in the Vines Center. This is an NCAA related regulation as well. And this is asinine.
Phantom brought up a good point that my father and I have discussed many a time. And that is that school divisions (at least many of them) don't pay for clinic or reimburse any mileage, hotel accomodations, etc... and that probably plays a major role in declining attendance..
Attendance was worse at Lynchburg last year than it was in the last couple of years in Hampton. Attendance was hurt some years ago by a switch in weeks- (forced by booking of HR Convention Center to the week of July 4th)...IMO it's never recovered.
Insofar at not "learning anything new"- Ain't buying it. Coached football for 17 years. Always picked up a new drill or two that we used with the programs I coached that following August.
Finally...I'm not sure younger coaches are as interested in the camraderie aspect that the clinic used to provide....What the reason for that is I don't know?