#1: iPads don't have USB ports, and
#2: There aren't any USB adapters that support printers, and
#3: iOS doesn't support printing via USB.

And the (IMO) main reason:

Apple doesn't want to load iOS down with the megabytes of printer drivers that OS X has. So they include ONE print driver - the AirPrint one. You either need to have an AirPrint compatible printer, or use your computer to fake it (ie, Printopia).