Gridcoin is kinda victim of its longevity here. Gridcoin exists since 2013 and for a few months in 2015 was among the Top 25 coins on coinmarketcap. Poloniex was a very popular exchange back then and, of course, Gridcoin was listed there too. Times changed, Poloniex declined sharply and acquired scammy reputation - many newer coins never even thought of being listed there. Obviously, they are trying to streamline their business now, to adjust to this very long bear market. If Gridcoin was a 2017 coin, most likely it would have never been listed there in the first place.