Just fashion. But because Bg5 was the move to create problems for black with direct threats.
However, for a while, poisonned pawn variation was considered at least equal for black. Nowadays, we are not so sure, see some games in Tata Steel...
Now we have Be3, Bg5, f3, h3, Be2, g3, a4, and others...
6. Bg5 has been analyzed to colossal depth and no refutation of the Poisoned Pawn variation has been found. When this started to become clear in the mid 80s attention switched to 6. Be3 thanks to the English and St Petersburg players and in the 90s 6. Be3 became tremendously popular. But since 2005 this golden age of 6. Be3 has somewhat passed too as both 6. Be3 e5 and 6. Be3 Ng4 remain solid for Black. Since 2005, 6. Bg5 had a bit of a revival, 6. Be3 is still investigated and new discoveries are made (like those venomous a3 lines recently), but many players have chosen other sixth moves instead to avoid all this theory, 6. h3 becoming also very popular.
The literature says that the "old" or "classical" continuation of the Najdorf was 6 Bg5, but that now the preferred move is 6. Be3
Can someone explain why that is so? (In more general theoretical terms)?
Thanks!