In other words, they had to support iOS 6 on 3GS, but for iPad 1 it was optional. Hence they didn't support iPad 1.

It's mostly strange because iOS 6 didn't really add any performance-critical changes, it was mostly changes to iCloud. Unlike iOS 4 that added multitasking, for example. Well, the new Maps.app is likely more demanding than old one, so perhaps they kept back the entire iOS 6 because of that.