Too busy with Dirt and F1 games probably. Also porting this game to as many platforms as possible (last I checked, you can also get it on Android/iOS/Switch)
I agree it's well overdue for the next one though. But the market is a bit crowded at the moment for circuit touring car racers with ACC, PCARS, GT/FM and the rumoured GTR3. I can understand why Codies is holding off as it doesn't make sense from a financial standpoint. They have practically no competition in rally (apart from the WRC titles) and F1.