[jaaron]

The developers were seeing out of memory issues on the iPad 1, thus the requirement for iOS 6. I believe there is still some hope they can get it to run, but ultimately, that didn't hold up the release:

https://twitter.com/TrentOster/status/276748438882902016

[jcromartie]

Yeah, the original iPad is the most memory-constrained ARMv7 (i.e. iOS 5-capable) iOS device out there.