If that's true, it's because the Giants game experience is miles ahead of the 49ers now.
I have been a 49ers fan for 20 years, but I can't stand going to games any more (when I go, don't live in the area so it's not so common any longer). Parking is $30 even 15 minutes away, getting in and out of the stadium is a nightmare, and Candlestick is a dump. You spend half the time at the game waiting in line for the bathrooms or at the concessions stands, and not in your $100 seat in the nosebleeds. Throw in ridiculous TV timeouts 4-5 times a quarter and I think people spend more time not watching football at the stadium than the other way round.
Contrast that with being able to enjoy SF all day, tailgate at a bar with your friends, and be able to get to the stadium in minutes, not the hour it takes to get in and out of Hunters Point. AT&T Park is beautiful, downtown, and new. And with the money you save on tickets, you buy food and drinks from local businesses.
If I lived in the city, I would be at Giants games all the time: it's just more fun, and that connects people with the team. The relative performance of the teams certainly helps, but I don't think it matters nearly as much as what it means to go to the games. Watching the NFL live is something I, and I'm sure tons of others, don't find enjoyable any more, many times over because of Candlestick, and that results in people no longer connecting with the team as much as they should.
[ Edited by 49toLife on Nov 1, 2010 at 7:59 AM ]