GTR use tablets paired (over Bluetooth) to standard paper ticket printers. I can't see why someone couldn't produce a printer with a higher DPI that can print Aztec codes with the same level of detail as on a bit of paper.
Obviously they looked around at what was available and the 'bog roll' printers were available now, or cheaper. I do find them a pain to store in a wallet, and they easily get creased. If folded on the code itself, I can imagine it causing some delays when attempting to scan them.
To be honest, by designing a suitable printer, there's nothing to say you couldn't print double sided on ticket stock designed for mobile printers.