I believe that it's because it's easier to see if anything's amiss by glancing at the dial, without having to actually read the number off it. This is not strictly relevant to car instruments, but it's why (along with the inertia systemic in aviation) glass cockpits still use steam gauges.

I could have sworn that there was a similar question (with answers) on http://aviation.stackexchange.com, but I can't find it now.