TR3
I don't pretend to be a professor on the topic. I am but a lowly 2nd year physics student and can't answer your question "why is c absolute?" with any great confidence. But as far as I know it is merely a theory which agrees with experiment. The experiment being the Michelson Morley expt. If you would like further details you'll have to ask and I'd be happy to describe it unless anyones got any objections.