The Raiders had the first shot at joining the Rams in LA and decided to head to Vegas instead. It was only after the Raiders passed on LA that the Chargers were able to move in.
My guess: the NFL gave the LA option to the Raiders so they could use that as leverage to get their own stadium, either in Oakland or elsewhere. Turned out to be in Vegas.
Then after the scheme worked for the Raiders the NFL figured it would work for the Chargers, too. I guess they didn't plan on Spanos gumming up the works. So now that San Diego has called Spanos' bluff the NFL has to figure a way to save face. Probably won't happen until Spanos sells the team - hopefully that happens sooner rather than later.
AlbaNY_Ram