The issue with any tech solution is that if it goes wrong, it can go wrong big time for everyone all at same time. With physical paper and wristbands you can do manual workarounds (for example double wristbanding in 2005 floods) or telling punters whose tickets were misdelivered to pick up a tixket at gate A. Also you are open to unexpected attacks.... so if I knew there was an Ethernet cable I could cut or a generator I could turn off, I might do so in the hope that everyone would be waved in. Simple physical solutions are often the best ones.