If ObamaCare was upheld, why strike down DOMA?

If you recall, the left was making a big deal out of the Supreme Court's upcoming decision at that time.  ObamaCare had to be upheld because that was the will of a popularly elected government.  By that same reasoning, DOMA should also be upheld, because it too was an act of Congress, which likewise represents the will of the people.

There may be a claim that times have changed, but if that's true, why not allow Congress repeal the law?

In other words, how can the Supreme Court remain consistent with itself and strike down DOMA, which is an act of Congress?

Proposition 8 was also an act of the people, which was expressed more directly.  Should the courts interfere with that too?

Both should be upheld.  Or the democratic process should be declared dead.