Bittrex is not based on ICO. The presence of Bittrex immediately made an exchange without conducting an ICO campaign. I think it's the same as INDODAX, Asia's biggest market. INDODAX also does not have its own coins. But it only supports new coins, like Tokenomy.
It is possible that a market like this builds its company by using its own capital, not from investors, so they don't need to promote their exchange with the guarantee of the coins they create.