guess because there is no offical classic release candidate.
this would btw be the case for serveral other addons too, I can list some which work just fine even though they are for wow 8.2+ retail.
also this would be affected by your issue about "switching between classic and retail" because you can not completely seperate between those two as "classic" is not really a vanilla client and its mostly the affected by which libraries and UI functions they use.
all handynotes plugins show up fine but I am missing the main addon:
https://www.curseforge.com/wow/addons/handynotes
guess because there is no offical classic release candidate.
this would btw be the case for serveral other addons too, I can list some which work just fine even though they are for wow 8.2+ retail.
also this would be affected by your issue about "switching between classic and retail" because you can not completely seperate between those two as "classic" is not really a vanilla client and its mostly the affected by which libraries and UI functions they use.
I would like to be able to add addons either way.
just a thought.