Why is it that the Coupe gets dirty SO quickly after you wash it?

I used to wash mine every weekend, and by Monday or Tuesday it was filthy again after driving about 50 miles and being garaged overnight.

I washed my BMW last saturday (8 days ago) and have done over 300 miles in it since and parked outside every night and it's still spotless, not a bit of dirt on it except behind the rear wheels.