Maybe it's just that a higher percentage of away fans drink so more likely to cause space issues. Imagine people who just buy a tea or a pie are more likely to go back in the stand and consume, which they're not allowed to do with beer. I dunno, I'm speculating. I've spoken to a few away fans who have moaned about it, it has been the case for as long as I remember.
Maybe we could open a couple of pop-up bars in the road at half-time and direct fans there but wouldn't be feasible before the game.