Because they don't have to discount the stock that they get in and have available to sell. They don't have to worry about depleting their standard inventory with a discounted unit.
I went through the same thing when I bought a 300C back in 2005 when it first launched. The 5.7L Hemi motor was on backorder so stealerships were selling the car at OVER MSRP. I did the same thing I'm doing now and found a dealer that said, "If you special order it and are ok with waiting 3 months for it to come in, then we'll sell it to you at invoice."
It's no sweat off their sack. They weren't planning on inventorying that car. They got to move a unit and still got the dealer hold back.