Actually, Democratic candidates won more votes than their GOP counterparts. But thanks to extensive gerrymandering, many of the Democratic House voters are all crammed into the same mostly-urban districts, handing them huge victories while Republicans win closer races elsewhere. I am represented in the House by Marcia Fudge, who was unopposed in 2012 and who has never received less than 80% of the vote in a general election. Meanwhile, my area's previous representative, Betty Sutton, lost her bid by a margin of 52.2% to 47.8% after her district was eliminated in Republican-run post-Census redistricting.
Ohio has 16 congressional districts. Pennsylvania has 18. Obama won both states, so one might expect that Democrats might at least be competitive in the House delegations. Unfortunately, thanks to how the lines are drawn, they only managed to win 9 of the 34.