Because congress doesn't reflect the popular vote, especially in a presidential election year.
When everyone votes, they vote democratic. There's only skew in the land area because democratic votes are highly concentrated in cities. Republicans tend to be more spread out. Stick gerrymandering on top of that and you have a really skewed congress.
Democrats & Republicans gerrymandered the districts until 2012 when the people voted in Prop 11 which allowed the people to vote for citizens to sit on committees for re-districting. It is still not great but, the outright carving of districts for incumbents is a bit less of a sure thing.
The problem with Republican candidates is there really are no good ones running.
I beg to differ, Dr.Ben Carson is a really good choice and a class act to boot. I hope he is able to continue to build support...it is rare to see an average, albeit very educated citizen, and a good man willing to step into the sewer of American politics to lead. I hope he has the stomach for it.