[vetinari]

It is matter of organization.

There's a single queue to multiple self-checkout machines. If one is stuck, you won't notice.

For classic checkout, there are queues to cashier 1:1. If one is stuck, especially the one you are waiting in, you will definitely notice.

This, together with limiting the number of open classic checkouts, nudges you toward the self-checkout. KPIs then show, that they are popular and cheaper, so continue will rollout.