"why can't I write web software in Python for the iPhone and iPad?"
Ummm... you can. If you write 'web software' in Python, all an iPhone/iPad user needs in order to use it is the URL. Both devices have a best-in-class web browser that would be more than capable of running your web software well.
Also, check this out:
Apple is putting effort into make web apps first-class citizens on their devices.