From what I read the CW did it because TVD was preempted in some markets on Thursday night.
For me it didn't really matter that they postponed
Supernatural because I'm so far behind and haven't caught up yet. And even when I am up to date, I rarely watch live on the night it's on - I always record the show and watch it later, the same way I do with almost everything I watch, including TVD. But I can certainly understand why the people who were looking forward to watching live weren't happy. Though I do have to also say that some of the reactions I saw on various entertainment sites went way, way overboard - I mean I even saw people posting death threats against CW execs because their show was postponed!
There's such a thing as being a passionate fan - but then there's such a thing as being a seriously disassociated from reality fan...