- Availability. There are police stations (kobans) and officers on the street everywhere, just a few steps away from wherever I may find something. Turning in a lost item has no friction. All it takes is a few extra steps. In the U.S. there is huge friction - I would need to go out of my way to go to a police station and turn in something.
- The general view of police and government as your "friendly helper" as opposed to enemies. Officers and public staff (at train stations, etc) are service workers there to help you. I can ask them literally anything - directions, store recommendations, etc - and it's totally normal. Turning in a lost item to some random officer on the street is just a normal. In the U.S., I try my best to avoid all people in police uniform. They tend to be unfriendly, not helpful, and their job is to see if I am doing something illegal - not to help me. While this may not be true, that is my perception. I wouldn't even trust them to actually take care of a lost item I give to them. And the fact that they're carrying guns doesn't make them look any more friendly/helpful.
The definition of what "police" means in Japan vs. most Western countries is completely different.