The coin developers themselves have to work to get them added. When you first create a wallet on openledger, at the bottom of the page it says "Add Coin", which allows any coin developer to fill out the information and get their coin added.

It is important for every gateway provider on the dex, to have a direct dialog with coin developers so if issues arise, they get ahold of the coin developers ASAP.

If coin developers haven't bothered with the dex, than the onus is on the coin developer themselves. But this doesn't matter, the dex still carries plenty of options. SMOKE for example, isn't even listed on coinmarketcap.com but it's a tradeable token with over 500,000 facebook users coming to EOS soon. So you could equally ask, why are there things on the DEX but not coinmarketcap.com either?

We don't live in a perfect world.. so we have to work with what we have.