Perhaps there's some headcannon to be had here, that when Ardana joined the Federation, it met all the membership criteria and had an egalitarian society, but as soon as Zenite was discovered along with its many uses, Ardanan society gradually backslid into the situation we saw in The Cloud Minders.
I am sure the Federation has great difficulty in working out what to do with members who backslide like this, its probably a very difficult decision to take to expel a member once they're in, particularly if such an expulsion would likely lead to them losing access to the Zenite permanently, and perhaps would precipitate an immediate invasion of Ardana by a competing great power.
We've seen issues like this occur with Hungary in the EU. Everyone agrees that Hungary has backslid away from the norms of democracy, but no-one can figure out what to do about it, and there's no formal process for expelling a country from the union.
I am sure the Federation has great difficulty in working out what to do with members who backslide like this, its probably a very difficult decision to take to expel a member once they're in, particularly if such an expulsion would likely lead to them losing access to the Zenite permanently, and perhaps would precipitate an immediate invasion of Ardana by a competing great power.
We've seen issues like this occur with Hungary in the EU. Everyone agrees that Hungary has backslid away from the norms of democracy, but no-one can figure out what to do about it, and there's no formal process for expelling a country from the union.