good question....
last year they did one other night for one week only. Can't remember what it was, but it was like a dj battle between Timo Mass and Fatboy Slim.
Maybe because the competition is just sooo strong on other days.
After all how many other parties do you know that take place during teh day on Mondays...
I'm sure that they make soo much money on that one day anyway to cover the week.