Because as usual for Apple products, it is overpriced. Fine. We can pay the extra money for the Apple simple design, but the fact is that it does not work nearly as well as the competition.
It is one thing to be an apple fan, but you need to know when apple does not make something right... so they will change that something out with something worth the consumer's money. My Logitech mouse is by far the best mouse I have ever used and it costs the same as the Apple mouse. It feels better in the hand, is more responsive, and does not have the infamous 'apple ball.' I am not being an apple hater here, just realistic.