Why is the Tartakower Variation of the Queen's Gambit Declined almost always predeced by 5...h6, while in the Orthodox defense an early ...h6 is almost never played by Black?
2 main reasons:
- The orthodox defense typically goes for a ...c6 structure and avoids ...b6 + ...c5. This makes Bxf6 a more viable option for White due to the more closed nature of the game. So, ...h6 could just end up wasting a tempo by nudging White to make a somewhat favourable exchange. Note that it's fine for you to play Bxf6 against the Tartakower too, it just works better against the orthodox.
- The orthodox could see Black doing the maneuver Nbd7-Nf8-Ng6 (especially if White goes for a Carlsbad by exchanging on d5). This requires control over the g6 square if White has a Qc2+Bd3 battery set up. If Black has already played ...h6 and then plays ...Ng6, White wins a pawn with Bxg6 fxg6 Qxg6.