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.
Comments
The 3GS supported iOS6 because Apple were still selling it at the time. They were not selling the original iPad.
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.