The big reason why the 49ers are "moving" is because the City of San Francisco has pretty much pushed them out.
Here's what the city has proposed and has also failed to do:
A) Candlestick Park is owned by the city of San Francisco and the biggest reason why its a dump is because the city has been using the money for other things. The team sued the city a few years over maintenance costs. Light poles have falled in the parking lot (which btw, totally sucks), speakers have fallen inside the stadium. Thankfully, those happened in the offseason and nobody was hurt.
B) The only place the city has offered is Hunter's Point, which isn't that far from Candlestick. Its a prime spot for a stadium, except it has one major problem. Its a superfund site. Its an old Navy shipyard and its one of the most polluted sites in the country. The Navy is working on cleaning it up, but the finish date is unknown. Any delay impacts the 49ers plans.
The 49ers have been trying to work with San Francisco for years and they finally decided to go with Santa Clara in 2006.
One big reason why they went with Santa Clara is because that's where their team facility and training camps are. The only thing they do in San Francisco is play 8 games in a rundown stadium that has fallen apart due the negligence of the site owners. The 49ers are a Santa Clara business, and have been for years.
The stadium is going to be built in that empty parking lot right across from the team facilities.
San Francisco was an option, the city failed to come up with a viable option (offering a superfund site with a cleanup depended on federal dollars and not offering to help pay for the new infrastructure costs and instead expecting the team to pay for that as well) forced the Yorks to play their hand. They don't have the time to wait for Hunter's Point to get cleaned up. Like I said, any federal delay in the cleanup delays the stadium. The 49ers can't risk that.
What happens if there is a major earthquake, can Candlestick survive it (its a major concern)?
The inability of San Francisco forced the 49ers to go to Plan B, which was Santa Clara.