Probably because PIN entry devices have a lot of security standards that you can't emulate on a touch screen.
It's a shame because basically everyone except the US has moved on to chip+pin, and the US will transition in the next few years, so this product won't be around for long.
You can have pin entry on a touch screen. Indeed Not as easy as not having pin entry, due to those security standards, but it is possible. Have a look at https://www.pcisecuritystandards.org/assessors_and_solutions... and filter on ‘touch screen, as entry technology. Populair name is pin-on-glass.
It's a shame because basically everyone except the US has moved on to chip+pin, and the US will transition in the next few years, so this product won't be around for long.