Because all other small ones have way less population.
Chandigarh is an exception, has 10 lakh population being most populous UT and yet top in UTs.
Chandigarh's management has been far better from any 1 million populated city so they should be given the credit. Also the general population is quite law abiding which of course helps the management.
Rash and drunk driving...
more... is a problem but even that is within control.