You had a perfect storm of -
1. Midterm election (inherently less favourable to Democrats)
2. Sluggish recovery
3. Republican base in a fever
4. Pendulum swinging back after outsized Democrat gains in the last two elections
A very large part of why Dems lost so badly was that Obama allowed his people to take over the DNC and tear down the 50 state strategy that had served the party so well from 2005-2008. With this infrastructure gone, most Democrats elected with its help had no support to help them withstand Republican attacks and onslaughts.
Had the infrastructure that Howard Dean had set up in 2005 and 2006 remained in place, Democrats may have held their losses to a more normal amount (around 30 seats in the House). Remember that Dems lost multiple seats that they should have been able to hold even in a bad year like FL-22, IL-17, TX-27, NY-24, NC-02, MN-08, etc.
The Democrats won in 06/08 not because of the 50 STATE STRATEGY or whatever but because Bush was as popular as cancer.
