Rocket League was a sequel to Super Sonic Rocket Powered Battle Cars which was a totally new game but born from the studio building VehicleMod for Unreal Tournament.
We're talking about hit games created specifically as a sequel to a hit mod of another game, and communication to the community of the hit mod that this is where the developers are going, and that they should move to the standalone game if they want to thank the developers for all that unpaid work they did on the mod over the years.