The reason why there isn't a Wurst version for Forge on Minecraft 1.16.5 is because Wurst is specifically developed for the Fabric modloader. While we did have a ForgeWurst project in the past, it never made it out of Beta and the last supported version was 1.12.2.
The main reason I chose Fabric over Forge is because Fabric is a lightweight modloader that is better suited for hacked clients like Wurst. It updates quickly to the latest Minecraft versions and snapshots, while also having built-in support for mixins that make it easier to "hack into" the game's mechanics.
Additionally, porting Wurst to older Minecraft versions can be time-consuming and can significantly slow down updates. Since there is a relatively small number of players using older versions, it doesn't make sense for me to put in the extra effort to support older versions on different modloaders, which even fewer people use.
I hope this clears things up for you! Let me know if you have any further questions.