user8291: Why is 
[fen ""]

1. d4 Nf6 2. c4 e6 3. Bg5 

almost never played? Isn't it a logical opening move? OK, it goes against the rule of "Knights before Bishops in the Opening", but is a general rule enough for this move to be virtually never played at top level?

user11609: Your bishop can be chased with h6, g5 and h5, after what white barely equalizes. It also gives black calm options, for example some c5 with Qa5 ans Ne4 ideas. It's just committing the bishop too early. So yes, it's typical position on why 'knights first' often works! 

user4588: After 1.d4 Nf6 2.c4 e6 3.Bg5, the position resembles the Tromposvky Attack (1.d4 Nf6 2.Bg5), where white often gives up the bishop pair in exchange for a lead in development and the center. For example, one main line goes 2....e6 3.e4 h6 4.Bxf6 Qxf6.
However, in the position after 1.d4 Nf6 2.c4 e6 3.Bg5, the move c4 doesn't fit well in this plan. Moreover, it weakens the dark squares, helping black's plans with c5 or Bb4+.


      [StartPly "2"]

      [FEN ""]
      1.d4 Nf6 2.c4 (2.Bg5 e6 3.e4 h6 4.Bxf6 Qxf6) e6 3.Bg5 h6




8291: <p>Why is </p>

<pre><code>[fen ""]

1. d4 Nf6 2. c4 e6 3. Bg5 
</code></pre>

<p>almost never played? Isn't it a logical opening move? OK, it goes against the rule of "Knights before Bishops in the Opening", but is a general rule enough for this move to be virtually never played at top level?</p>
 11609: <p>Your bishop can be chased with h6, g5 and h5, after what white barely equalizes. It also gives black calm options, for example some c5 with Qa5 ans Ne4 ideas. It's just committing the bishop too early. So yes, it's typical position on why 'knights first' often works! </p>
 4588: <p>After 1.d4 Nf6 2.c4 e6 3.Bg5, the position resembles the Tromposvky Attack (1.d4 Nf6 2.Bg5), where white often gives up the bishop pair in exchange for a lead in development and the center. For example, one main line goes 2....e6 3.e4 h6 4.Bxf6 Qxf6.</p>

<p>However, in the position after 1.d4 Nf6 2.c4 e6 3.Bg5, the move c4 doesn't fit well in this plan. Moreover, it weakens the dark squares, helping black's plans with c5 or Bb4+.</p>

<pre>
<code>
      [StartPly "2"]

      [FEN ""]
      1.d4 Nf6 2.c4 (2.Bg5 e6 3.e4 h6 4.Bxf6 Qxf6) e6 3.Bg5 h6

</code>
</pre>
