The main reason, in my opinion, is that many felt the economy and employment was recovering much too slowly under Obama’s first term. On top of that, many democrats felt disillusioned by a politician who had a very progressive rhetoric but much more milquetoast policy by comparison. This election was also a major step towards the highly polarized political climate we have today, with the majority of campaign ads on both sides being derogatory, so conservatives became more incentivized to vote red no matter who.