Summary: Cumulus radio, under management of the Dickey brothers, despises the conservative talk radio that brings them so much profit. Hannity ditched them. They replaced Hannity with loud-mouth Savage, and then the audience for Savage was about half the size of the audience Hannity had been getting in the same slots.
That isn’t why his ratings have tanked at all.
Read the responses in this thread and you will know why.