But the DC case involved whether the city could, under the law, hold a public vote, or be required to. In California, such a vote is provided for in the state constitution. So they're two different issues. In DC, the issue was procedural; in California, the issue is constitutional.
Further, the DC case would have applied only top DC, but if the Ninth strikes down Prop 8 on constitutional grounds, it will apply to Alaska, Arizona, California, Hawaii, Idaho, Montana, Nevada, Oregon, Washington, and Guam plus some other Pacific islands. If the Ninth upholds that lower ruling striking down Proposition 8 and the Supreme Court doesn't hear the case, the decision will become 'law' in all those states.
I'd personally rather see a case in another circuit or two, decided in favor of marriage equality, coming up at the same time, but we deal with what we have.