I am not a strong player, so I could be wrong, but my understanding is that 1. d4 d5 games tend to be more closed than 1. e4 e5 games, because it's less easy for the center pawns to get taken (because they are defended by the queens).

If any stronger player wants to comment, I'd be interested to know whether this is indeed the main reason 1. d4 d5 games tend to lead to more closed positions.