In addition to @nmei’s insights, I think Poloniex may attract more sophisticated traders because of its advanced features like margin trading. Those sophisticated traders may have higher awareness of NBT’s ability to be used as a hedging tool against BTC price depreciation.
http://coinmarketcap.com/currencies/nubits/#charts shows the daily trading volume. If you set the window to look at the period after our exchange hackings (March 1, 2015 - Present), you can see that trading volume has been growing at a healthy clip since June 2015.