The reason they took the case was to decide on that issue. There was no intention to actually rule on gay marriage. This wasn't some sort of cop-out, it was exactly why they took it. A lot of people, on both sides hoped it would rule on more, but it most likely wasn't going to happen. The ruling would almost certainly be either, they had standing and the Court of Appeals decision (ruling it unconstitutional) is valid or they didn't have standing so the District Court decision (ruling it unconstitutional in a slightly different way) stands. No matter what it was not going to have an impact outside of California.Not to get pissy about minutiae, but I'm not thrilled with the way the Supreme got out of giving a real ruling on Hollingsworth.
As for Windsor, the case had some differences. First, BLAG is part of a US government body that passed the legislation. Second, the refusal of the Treasury Department to fulfill the judgement on behalf of Windsor was important as it meant the Executive was still involved in the case. The judgement also goes on to state that the Windsor case is irregular and is treated as such.