Because, whilst I really like the design of the new Mac Pro, it is for the wrong market, if it had a haswell chipset, and allowed standard PCIe GPU's (crossfire and SLI support also) then I could see it as a prosumer device for home use and gaming .
But as an office/studio work station, it makes little sense.
So I wonder if a lot of people who had been holding out are now looking at custom hackintoshs for their pro needs.
Thats because it is now clearly aimed at High End Video Editing.
People doing Gaming and Home Use hasn't been the Target Audience of the Mac Pro. It has however been adopted by people in that market space. However that doesn't make those users the target for the box. Lets face it the number of threads in the past about people wanting to buy a Mac Pro because the case styling is nice, but only want to load Windows and Games up on it.
Apple are clearly pushing the Mac Pro upwards with this new one, away from ProSumers and individual Pro Users and more towards Studio's etc where the absence of the built in drive bays isn't an issue as will be be stored centrally on the Big Enterprise SAN.
Either these people will
1.) Keep there existing Mac Pro's as they do the job
2.) Move to Maxxed out iMac's
3.) Go Hackintosh route
4.) Move off OSX and change Platforms.
Even Steve used to say that wasn't trying to target all users, they would concentrate on there target audience, and if that didn't suit you basically tough! You either fit the Apple model or you didn't. Home Users and Gamers do not fit the model, as Apple would target you with an iMac instead.