More notably, more serious problems may require more staff. If the nurses are busy cleaning up bio material, you get to wait a little longer too before you get triaged.
The incentive is not to drag your wait out, so much as to have the minimum staff needed to keep patients stable. Stabilizing people in critical condition is literally the one job of the emergency room.
On the other hand, people go to the ER for all kinds of reasons that are more urgent than urgent care, but less than true emergencies. Sometimes, they even go because they can't be denied admittance for lack of ability to pay, when a general practitioner at regular practice would have been a more appropriate physician to see.
All of that said, a 10 hour wait either means there's an absurdly high number of non-emergencies, or they're facing severe shortages (self inflicted or otherwise).