What Curse wants for modpacks is them to be using a manifest and modlist files, you can do this when exporting a modpack as they will be created/appear in the zip file with the selected things of configs, mods, saves and so on.
Just upload the exported modpack to the curseforge site and your good to go. No configs/mods/saves manually put into the folder and zipped up, I learned that the hard way. They want it to automatically be sent to users when they choose to download the modpack basically through Twitch.
The only other thing I can think of is maybe the licenses of the mods but I'm not sure as I've not encountered this being the case when uploading modpacks.
Niche Mod Finder/Acknowledger/Tester/Player. Fan of Datapacks, Rift 1.13/1.13.2 Mods, Fabric 1.14 Mods and interesting underrated or new 1.12.2 and below versions of mods. :) Testing 1.13.2 Forge and Fabric 1.14 mods mostly though.
I sometimes create the odd modpack or adventure map once an idea comes together.