It wasn't really necessary anymore to have coded sirens for calling firefighters anymore, as the main purpose for coded sirens is for fire calls. After VFDs started using radios for calling volunteer fire fighters to the station, they started to not become as Nessesary.
Besides, many fire departments already had or have a siren to call fire fighters still even though radios are more common. Many sirens are very expensive to buy, and it would just be easier to repair the siren they already have. It's a classic case of "if it ain't broke, don't fix it".