I don't think some people realize the scale of DTRS (Colorado's trunked comm system). A quick look shows DTRS currently has ~3,400 unique talk groups and ~300 towers. It also averages abound 9M 'calls' a month. That is one of the largest in the country.
Encryption requires installation of additional software and/or hardware in radios and dispatch consoles - every single radio/console involved in the encrypted comms requires an upgrade. Additionally the encryption keys need to be managed and changed frequently.
Also, the various 'scanner apps' typically delay their re-transmissions over the Internet by 5-10 minutes. Further, many of them do not include SWAT and 'sensitive' channels.
What is my joy if all hands, even the unclean, can reach into it? What is my wisdom, if even the fools can dictate to me? What is my freedom, if all creatures, even the botched and impotent, are my masters? What is my life, if I am but to bow, to agree and to obey?
-- Ayn Rand, Anthem (Chapter 11)