It's need vs. cost.
A lot of police departments have a need to be encrypted. Older analog radios made it difficult to do. With more and more departments upgrading to newer radios and digital systems, enabling encryption suddenly becomes as easy as a check box while programming the system. So, as equipment gets replaced be it because of narrow banding, P25, LTE, whatever, you will likely see encryption become more prevalent.
Need:
As was stated, most fire departments don't have the need for encryption. Actually, it benefits many of them to not be encrypted. Likely you will see most fire ground frequencies stay the same. Same goes for wild land fire fighting. Conventional/simplex is still commonly used, and analog is the standard, so unlikely you will see encryption any time soon.
Other areas you likely won't see encryption until the need for technology changes things:
Marine VHF
Aircraft VHF/UHF
HF/short wave
12.5 KHz analog radios are going to be around for a long time. Eventually we will see the FCC push for 6.25KHz or better, and then these folks will be pushed into the digital world.
I don't see Marine VHF, Aircraft, or similar users going that way any time soon. FCC has made no plans to narrow band any of these services.
Free and easy access to your local police department communications will likely go away in the next few years, be it them going to digital or LTE, it's going to happen.