Giants ownership is partly responsible as well. In the early 90s, the Giants were threatening to move to St. Petersburg, and the A's went out of their way to convince them to stay in the Bay Area, going so far as to giving the Giants the exclusive territorial rights to the San Jose area so they could build a new stadium there.
Even though the Giants eventually sold to new owners who stayed and built Pac Bell with their own money, they retained the San Jose rights that the A's had given up and then used that to kibosh the A's potentially moving to San Jose themselves. No good deed goes unpunished.