Because former Blue Dog Steve Israel has as his goal the resurrection of the Blue Dog Caucus or something very similar, even at the expense of the possibility of Democrats taking back the house:

Last week, when MoveOn and PPP released their much buzzed about polls showing how Democrats could pick up many seats, the first thing I noticed was that these were all the Steve Israel seats they had polled. I spoke with them and asked them to poll some of the districts that the DCCC studiously ignores, districts we’ve been covering here at DWT and where Blue America has some great candidates. This morning MoveOn and PPP are releasing new polling data for some of those districts.

Outstanding, at top Blue America races where Lee Rogers is ahead of Buck McKeon (CA-25), Paul Clements is ahead of Fred Upton (MI-06) and Jason Ritchie is ahead of Dave Reichert (WA-08). In other districts, like FL-27, where Steve Israel and Debbie Wasserman Schultz have actively discouraged Democratic opponents, there is a clear indication that if Pelosi manned up and fired Israel and cleaned out the nest of self serving incompetents who run the DCCC, the Democrats would win back the House hands down in 2014. In many of the districts where there are no Democrats– thanks to Israel’s agenda– voters see no alternative to the GOP incumbent but there is a clear indication that a Democrat could campaign and win. In FL-27 where Wasserman Schultz has been protecting Ileana Ros-Lehtinen for years, if an election were held today, an unnamed Democrat would beat her 47-45%. ………

You can read the rest at the article, but basically, the DCCC will drop big bucks on ConservaDem long-shots, and ignore competitive races where real Democrats are running against vulnerable Republicans.