Why would they? There are other coins that you can point to with more consistent volume that are of zero interest to them.
It would have to be dominating volume for months on end for them to consider it.
They added ETC so months on end is not really necessary. Like any other business they're looking to make money and they could do that with XEM (I'm pretty sure poloniex is pretty happy with basicaly being the main exchange for XEM). You're completely right that the volume is very low for extended periods of time but XEM does have really high volume sometimes and it's not like it costs them a lot to add it.