Why would they add Darkcoin? Didn't Finex de-list Darkcoin because nobody used it? My theory is that the shady Russians wanted some type of mixer for their scandalous activities and aren't convinced a monero wallet will ever come out.
Isn't the dash backend much more similar to bitcoin than Monero? Ease of implementation of a clone might have had a lot to do with things.
Not really. I would say they intentionally didn't add Monero, because ETH is not a bitcoin clone and they added it anyway, they just probably didn't see any reason to add Monero.