I'm active on NationStates, and I don't do anything in the World Assembly beside vote, and I have no reason to vote so I wouldn't bother if I wasn't otherwise playing the game. People are active primarily for other reasons, the World Assembly isn't normally the primary reason.
Furthermore, this game is quite different to NationStates, with the two games sharing very few similarities. Adding a World Assembly similar to NationStates wouldn't add much to the game imo, given how if there are meaningful changes to a nation as a result of the legislation passed, there would be a few annoyed people.
I get you've said that it should be voluntary, but it just wouldn't fit in here.