I think police motorcycles are less useful because really, what can they do besides traffic stops which don't result in an arrest? I see a few, but for about the same money probably, a municipality can have a patrol cruiser which can transport arrestees, carry tons of gear, and most important, have an always-on computer, which is almost essential to modern police work.
What ar the advantages of a bike, except that maybe they can lane-split on highways to chase speeders?