My own personal reason is that I travel about a lot and I am rarely in Donny for more than a couple of days at a time.
I think the traditional mon-fri, 9-5 working week is slowy dying out. People now tend to work more on an ad-hoc basis (shift work, 0 hours contracts, internet businesses etc.). The days when you all worked in the same place and the weekends were 'leisure' time is slowly changing.
When I was a regular I went for the atmosphere, friends, drinks, singing and shouting random things at men 3 times your size. The success of the team never really mattered to me I as I always knew we were going to stay at the same level. I enjoyed the characters at the club and the interaction between players and fans. I don't honestly know how much of this still applies at the Keepmoat because I have only been to a handful of games but I suspect is has lessened?
I read on another thread about the lack of young fans at the Keepmoat. If this is true, then this is definitely a worrying aspect. I don't know what the prices are like at the rugby but football certainly excludes youths in my opinion. I'm talking about those who are 16-21 who arn't in work or don't get enough money off their parents to go to a match on the weekend. I think if you lose em in at that point it's hard to draw them back in.