It’s what @zenvelo said. Most people reach with their right (the majority of the population is right handed). The person inside uses their right, and the person outside. It’s rare a shop owner opens for the day from the outside.
Confirmation bias might play a part also.
Depending on the type of door, it might be one door is a simple twist lock, and the other door has slide locks into the floor and above the door. For this type of double door, unlocking the second door is more effort and more troublesome, but this type is usually on residential not commercial. Commercial might have locks that anchor to the floor, but usually they are a twist lock control also.
I find it annoying that one door is left locked. That shouldn’t be the case in a commercial situation. Sometimes it’s a mistake, and they just failed to twist all the locks.