Some public places do have rooms designated for adult diaper changing but you usually need to have proof of a disability to get to use the adult diaper changing facilities.
I personally just use a regular stall and stand up and back my butt up against the stall wall to change my diaper in public places.
I keep diapers in a backpack and my other changing supplies and do a quick change in a public restroom when away from home and I've never had a problem.
If more places did have public adult diaper changing facilities it would be great but it costs money to put those facilities into place so it's really up to the business whether it would be worth the cost to install a public adult diaper changing facility or not.