European football can only be played as long as the goal posts are above the water. If it's very rainy they call it "water polo".

On a more serious note, they don't stop it because of rain, but if it's really pouring buckets they do.

It is all to do with the safety of players and whether or not the water levels would affect the game.

I've not heard of a football game in Europe (at least in England espically) being delayd because of the rain. What would normally happen is that the ref will call a pitch inspection roughly four hours before the kick-off time, make a decision and that's it.

Occasionally the ref may stop a match if the rain during the game is playing too big a role in the match outcome itself.