Why's that? Do you mean in dollar terms? Because it's (for many people) more expensive to be a fan. NFL ticket prices for many franchises are very high. For many, it's more expensive just to watch the games on NFL Network or ESPN or even Fox/CBS, because of commercial cable packages and in some cases, because of the switch-over last year from analog to digital. DirectTV will lose out big time because of all the NFL Ticket subscribers. And few have discussed the impact it will have on the college draft.
It took MLB a long time to recover from the 1994-1995 disaster, and the NHL was severely affected by the 2004-2005 stoppage (though the NHL made an amazing recovery). The NFL talking heads are projecting an even bigger disaster to the fan base if there's a work stoppage.
I blame much of this on Roger Goodell ( a commish I otherwise like), who has sat back and watched this unfold and clearly wants to see the owners break the backs of the union.