That can be an advantage of "timeless" industrial design; it doesn't matter how long it takes to bring it to market. The G5/Mac Pro cheese grater hasn't changed since 2003, but what's ten years when you're borrowing from the 1960s?
It also helps that the vast majority of Apple's customers aren't even aware of what the Mac Pro is if they have even heard of it before. Many Apple stores don't even have them in the drop-in area anymore. I can't count how many times people have assumed I own a MacBook Pro and just said it wrong.
If it was a higher profile machine it would have gotten a redesign by now. Hell, the only machine that spent more time in its final form factor was the Xserve, which literally can't be redesigned because there are only so many ways you can build a 1U rack mountable server.
