For the same reason you need a licence in most places to install fire and security systems. If you make a mistake, people can die.
The proposed legislation:
https://legiscan.com/CA/text/SB244/id/2832427
(ii) A product or component of an “alarm system” as defined in subdivision © of Section 7590.1 of the Business and Professions Code. Code, including a fire protection system, as defined in the California Fire Code.
That makes sense then. When you search alarm system anywhere else you will get results for security systems which these days are simple consumer devices that lock customers into sketchy proprietary “ecosystems” that require subscription services that prevent users from operating and repairing by themselves.