Part of that is because they are more honest and ethical than elected officials.
Check it out:

A series of national surveys conducted periodically by Gallup since the 1970s indicate that Americans have consistently rated the honesty and ethical standards of police officers higher than local officeholders, state officeholders, governors, and members of Congress.

In fact, according to historical data Gallup has posted online, in none of these surveys has any group of elected officeholders ever been rated higher than police officers for their honesty and ethical standards.