Well, the bigger issue with San Diego was the location. Mission Valley is cold as fuck in the winter, and the only place the city was willing to work with them on a new stadium was going to be there on the same property as their (then) current stadium. It also isn't central like Petco is, and that's what Spanos really wanted.
That being said, it was a case of neither side wanting to really budge, and, really, the city admnistration not being as enamored with the Chargers as they needed to be to make a deal. Spanos seems to be a nice enough guy, but he lacks the developer's instinct (ruthlessness?) that Kroenke has. St. Louis wasn't going to full fund a new stadium after being on the hook for the one that the Rams left behind (and that was only a little more than 20 years old), and getting a perennial also ran like the Chargers wasn't going to be an equal trade for the marginally better run Rams. Putting out $400million for the partial cost of a new stadium was already a stretch and a last ditch for St. Louis (and the state of Missouri), and it was more contingent on keeping the Rams and their infrastructure in place. Spending that and other tax incentives to get the Chargers there would not have been particularly cost effective trade off for the local economy. In fact, I think the only reason they even tried it is because they knew Kroenke was stringing the along and they wanted to force his hand.