Power Ledger got listed on Bittrex because they gathered 10 times more USD in CO then SunContract did. IS simple as that. More ICO investors higher volume and Exchanges care abut volume.
I agree, more volume means more fee for trade. Of course the project, the team, the technology, the community etc.. are important for the exchanges but the most important thing is the money which they will earn from the trade of the coin.
But there are also many coins which have low volume than SCN at Bittrex.