it may not be best choice from your own view but you can not forget that trade.io invested $5million to support wpp project during fund raising, between i dont see any thing wrong in including trade.io as an additional exchange for users of that exchange, the wpp team already mentioned that subsequent listing announcement will follow, maybe the next listing will be coinbene or houbi korea, every thing will happen in due time.
As mate said above third exchange could be coinbene or houbi korea, these are the approved exchanges.
We can expect team will try to list on other big exchanges after long time but for now they are also following market so just buy at low price and HODL.
By now you should know it is not easy to satisfy human, i have read different complaint in their official telegram chat about DigiFinex not real volume, only bots are trading blah blah blah, now Trade.io will list wpp on 17th still people complain about low volume, like what the poster above has mentioned, it could be that trade.io has real volume and no bot trading to inflate trading volume, don't forget other listing will follow subsequently, people always expect good news at given intervals, so best idea for the team not to list in all approved exchange at once.
Whatever the wpp team does is the best they must have considered it so investors just wait for the best moment come in the near future
maybe some people believe that WPP will go to $ 0.1-0.5 in the next few years