Here's my theory. Apple has made its reputation in the 90s with the film/graphics/illustration market, and that's where the Power Mac/Mac Pro was a critical product. Once the switch to Intel was made and laptops started catching up in speed/GPU power, less people needed a fixed tower just for Photoshop/After Effects/Maya, they just put those apps on a laptop. Moreover, the professional market is always smaller than the consumer market, and consumers don't want desktops, so Apple just went where the money is.

They got rid of Xserve and they'll likely get rid of the Mac Pro if demand falls significantly.