Re: good grief
Because only using numbers would give far fewer combinations for a given length, and combinations are perhaps easier to remember?
Anyway, your knowledge on Outside UK is obviously limited. Portugal started (at least the registrations you see now) with AA0000, then went 0000AA, and then 00AA00. This year they then went to AA00AA. Denmark and Norway both use(d?) AA 00000, I think in Norway the letters denote region whereas in Denmark it hasn't since early 70es or so. Spain a few years ago changed from a regional system to what appears to be national with the format AAA 0000. So there!