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

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

I realise it's kind of ridiculous to complain, but it's also ridiculous that a 14 year old game won't play on a few years old tablet, and I'll need top urchase this year's model if I want to play said 14 year old game.

If you can manage to setup a DOS emulation layer on your iPad 1, you'll find out that the 14 year old game would play just fine.

The new, just released, native version of the 14 year old game, however, will not.

Nor on my iPad 3, which I have deliberately not 'upgraded'... oh well, I'll wait for BG2.

I pasted this above as well, but they're working on iOS 5 support:

Baldur's Gate: Enhanced Edition for iPad is currently English only, with additional languages coming in a free update. The iPad version also requires iOS 6, but iOS 5 support is coming soon!