Why 5...h6 in the Tartakower Queen's Gambit Declined and not in the Orthodox Variation? 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.