Because they had an excellent advertising scheme.
Before I start this let me say I VOTED NO!
Now,
They focused on kids being taught about gay marriage in schools, while this is not a common practice, under the California constitution it is allowed. I think this made a lot of people uneasy. Personally I have no problem with the LGBT community, as in regards to ME. While I don't have children I do question if my opinion would have changed in regards to the prop. It is a very touchy subject for a lot of people, and I am sure marriage will be on the next ballot and will pass.
We will have to see. In any case no one should be told they cannot marry.