The big reason is that they thought they could buy the elections by pouring in overwhelming amounts of money into them for ads. It was the ground game that won it for Obama and many Dems who worked with him. In Ohio, Romney pumped something like five times the money on ads, but he only had 20-30 offices in the state. The Obama campaign had 103, as I understand it. Ads don't win elections. Getting out the vote does. The Dems did that better, and the suppression attempts and the ad blitzes only helped them in their GOTV efforts.