Sicilian Najdorf 6. Bg5 Nbd7 line [fen ""]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Bg5 Nbd7


1) Why 6... Nbd7 is not played by many players? Why they prefer 6... e6 only? I could not get the answer for this. Requesting experts to answer this question with detailed explanation &amp; examples. Thanks in advance.
6.Nbd7 is played and often the lines transpose.
If Black continues to try and avoid e7-e6, his position, though solid, is less aggressive than the other lines.

Fischer said something like: 


  "The Sicilian is not a defense, but rather a vicious counter-attack!"


so with 6...e6 7.f4 Black has options like 7...Qb6 entering the Poison Pawn Variation, or 7...b5 entering the Polugaevsky Variation which are considered some of the sharpest variations in chess theory.  Why spare White the worry of having to deal with either of these lines?

6...Nbd7 is playable and probably leads to equality as well but Black is delaying his counterplay if he avoids ...e6 for long.   Some issues are that Black would like to play Qc7 and then has to worry about Nd5 when if Nf6xd5 exd5 results in a favorable structure for White.