San Diego is trying like hell to get more involved in the Tijuana/Mexico market in terms of TV revenue, which is about the only direction their market could grow locally.
On the US side of things, they are ranked 30th in terms of market size nationally, while Milwaukee is ranked #38. Being as close to LA as they are actually limits their market size in terms of TV revenue, much like Milwaukee is limited by being close to Chicago.
The difference is that San Diego does indeed have a decent chunk of built-in revenue with properties they either own or have a stake in around Petco, while Milwaukee doesn't. Plus having Seidler throw money at the team for years once he realized he wasn't long for this world. But, that extra billionaire cash has dried up and the extra property revenue isn't nearly enough cover what they currently have on their payroll books longterm.