There's money in handing out citations and the boot, and drug arrests.
That could be some of it, but also the officers who do these tasks may not be sworn gun-toting officers. In Seattle, the parking enforcement isn't done by armed officers. I have no knowledge, but suspect they don't even have arrest powers. It would be kind of embarrassing as well to have them show up at your door to investigate your break-in and ask if they can borrow your gun.