I hadn't thought of it like that before... And this is spot on. Two of my good friends didn't want to watch the games with me at home last cup since it interfered with their shows and evening plans. This year, however, I got a bunch of friends from work to watch in the cafe during work. They loved the first US match and actually made time to watch the US-Portugal game last Sunday...
On a side note, I think another problem to achieving more popularity is the nature of the game. Two halves of non-stop action means no time for commercial breaks. Major networks certainly can't be a fan of that, unlike for other US sports with plenty of breaks in the game.