These things don't design themselves and engineering resources are limited. Most manufacturers have a few year lag between the introduction of a new model and the arrival of specialty variants like convertibles, GT350, ZR1, M cars, Raptors, etc.
I don't have any inside information, but assume Ford is working hard on the 14th gen F-150. It won't be until that gets pushed out the door that Ford will allocate engineering resources to the Gen 3 Raptor.
Most of the time manufacturers drop the variants altogether, but sometimes they continue selling them with the old body. For example, to avoid gap years, BMW often sells the old-body style convertible alongside the brand new model.
As for the Raptor, it was only out of production for two years, 2015 and 2016. I suspect the same will happen this time.