This practically hands this seat to Mia Love, black, female Republican. Dem’s poured outside money to keep Mia out in 2012 (she lost by about 500 votes), but Matheson probably saw the writing on the wall for 2014, and figured he would avoid tarnishing his name with a loss. Matheson remains electable when the winds are blowing his way - but that might not be the case in 2016 either (after 8 years of Obama stinking up the Dem brand).
Matheson was not scared out of the race..he is looking at the Senate. Remember he was reelected last time in the state giving Romney the highest margin of victory.
He looks at Hilary running in 2016 and sees an excellent chance of being the next senator.
This was not good news for Conservatives, a RINO will take Mathesons seat and a Dim will get Lee's seat in 2016...