I see you got an answer but I'll jump in with my perspective. It's all about the environment in which the Macs are fitting. In 2008 the software environment was very different. We had software which did all the computation on the CPU and the user interaction on the GPU. Back then we needed beefy CPU options and we got them at good prices (specifically dual CPUs)
Now it's different. Apple were moderately smart in that they saw a decline in the role of the CPU, and increasing GPU computation. Graphics software has moved very rapidly to embrace GPU computation. On the face of it Apple made a smart move, but actually they didn't back it up with joined-up thinking. According to our vendors their GPU drivers are the worst in the business. Some of our software's GPU features just don't run on Macs, or nMPs for this reason. They overpriced the nMP with Xeons and ECC RAM which are a waste of money in a single CPU system (IMHO). They gave us no upgrade options, which is just infuriating in a fast moving industry where every client wants double what we delivered six months ago.
So thanks to that, our software vendors say nobody buying nMPs, which makes working through buggy drivers an unprofitable cause for them. A major graphics software vendor (one you have likely heard of) told us they don't even have a Mac Pro to test on. As a result, we get more work done on older iMacs (with nVidia GPUs).
Meanwhile, in the PC world you can pick up cheap boxes with 4GHz i7s and four or more cutting edge GPUs which are actually fully supported by software. Apple thought they were being bold and revolutionary with one CPU and two GPUs but they bungled it, and have been completely leapfrogged by PCs with vastly better performance/price ratios.