Some comments come accross a bit 'naive'.
Any medium or large corporation will drop what used to be it's core business if there are new ones more juicy.
The macpro line may still be profitable, but that never was a reason for a company to still do it. It may be the 3d or 4th more profitable segment.
They may want to drop a market which shrinks (or to be more precise, that is surrounded by segments that outperforms) and refocus the research and support $$ towards what will be the next thing. (tablet or whatever).
I think Apple realized that the consumer market is soooooo much larger that pro does not need to be in their vocabulary anymore to grow. Quite the opposite.