Is it just because of the low megahertz clock?
No, it's because it's a workstation card. The hardware and driver are optimized for running CAD packages and rendering software, not games.
Also, some workstation cards have
DMS-59 connectors, which is not the same as a DVI connection even though it looks a lot like one. It can be inconvenient for consumers because as far as I know, DMS-59 adapters don't come with HDMI ends which means you have to use bulky DVI or VGA connectors that don't carry audio.
In my experience, people building their own CAD workstations tend to put gaming cards in them anyway because they're about half the cost of a workstation card. Right now, a top of the line 690 will run you 1k. That
Quadro Force workstation card pushes 4k. Incidentally, if you were wondering why you couldn't find that card on newegg before, it's because you were looking on the
wrong newegg. Welcome to the enterprise world where you can buy $5000 10 core processors, cases come in two flavors: rack mounted or hanging off the back of a monitor because it's a thin client running vmware, and
this thing.
