Queen's Gambit Declined: Why 1.d4 d5 2.c4 e6 rather than 2..Bf5? Why has it become standard to decline the queen's gambit as below:

[fen ""]
1.d4 d5 2.c4 e6 


which blocks the bishop, rather than: 

[fen ""]
1.d4 d5 2.c4 Bf5 


?

By analogy to the king's gambit:

[fen ""]
1.e4 e5 2.f4 Bc5
First, as @RemcoGerlich pointed out extremely well, the analogy is not exactly the same:


  And the analogy to the king's gambit is flawed because there 3. Kg3 isn't a move ;-), and 3. fxe5 is not a threat because 3. ... Qh4+ is devastating. That means in the QG white actually has pressure on the d5 pawn, and in the KG he doesn't, at least not yet. Someone should combine these comments and make an answer, I don't have time... –  RemcoGerlich Sep 9 at 7:38


But the most important reason why this is not played is that there will be too much pressure on the d5 pawn by the natural move Nc3 and even Qb3 in some cases. This means it's not a good time to start developing pieces, not to say breaking the "principle" of knights getting developed before bishops, as the pawn has to be supported.

A good example was provided by @kahen:


  @AlwaysLearningNewStuff 1. d4 d5 2. c4 Bf5 3. Qb3 e5!? is quite messy though. White gets the upper hand more easily with the simple 3. Nc3 e6 4. Nf3 or 3. cxd5 Bxb1 4.Qa4+ Qd7 5.Qxd7+ Nxd7 6.Rxb1 Ngf6 –  kahen Sep 8 at 21:15


Taking a look at the mentioned lines:

[StartPly "4"]
[FEN ""]

1. d4 d5 2. c4 Bf5 3. Nc3 (3. Qb3 e5!?) (3. cxd5 Bxb1 4.Qa4+ Qd7 5.Qxd7+ Nxd7 6.Rxb1 Ngf6) e6 4. Nf3 {This is the main answer to black's second move. Notice how the black bishop can be easily exchanged by one of the knights if white wants to (and gaining the bishop pair in the process), and also how the b7 pawn can be subjected to attack by Qb3, putting further pressure also on d5.}


The thing is, this line gives white too many options concerning developing with tempo and forces black to catch up to him on every move. While not directly losing, it's understandable that modern chess players prefer to avoid such problems by leaving the bishop on its starting square, and seizing the initiative otherwise.