Very long time reader. First time poster.
Here's my guess...
The recent iMac and the Mac Mini are now seriously cutting into traditional Mac Pro areas. Mac Mini is a pretty decent server for small & medium business and now the high end has decent graphics for a mac gaming rig. The newest iMac is a beast. A 27" IPS screen and plenty of horse power for 90% of the graphics/design market. I'm a film editor on TV shows with 10+ editors, and it's not uncommon to have half the edit bays as iMacs these days. Plus with Thunderbolt, iMacs can capture and output uncompressed HD video and attached to the kind of raid drives that used to require a Mac Pro. In a year 90% of the things you needed a PCIe slot for will be available for Thunderbolt. The exception to this of course being high end graphics upgrades, but really people who are 'serious' about gaming on a Mac is a pretty tine market, compared to PC gaming or gaming consoles.
So the Mac Pro is becoming cannibalised for the mid to high end by the iMac and the Mac Mini. So who do you make a Mac Pro for? It becomes a machine for very high end. People who need Unix and the horse power only Xeons can bring. That means medicine, biotech etc, industries that used to used mainframes or workstations. In my industry, 3D & VFX workstations utilise every core a machine can spare and Color grading suites such as Davinci run best with an external chassis and as many Graphics cards as they can get. These are people willing to pay big money the most power, so if the Mac Pro is going to have a profitable market, it has to be the high end.