The Chargers are a lot like the Vikings in that they are franchises that have been way too successful to have not won a Super Bowl yet (yes, both won championship titles in the AFL/NFL days). The reason why, terrible history of management. Look at the history of both teams when either had a chance of making serious moves to help win a Super Bowl, but somehow let great talent (players and/or coaches) get away instead.
Spanos might be a billionaire, but he's in the NFL for the money only, not Championships like another billionaire, Jerry Jones. Luckily for the rest of the league, there's a salary cap and that Mr. Jones proved to be a foolish GM.