Because " already get listed " means that a token is available to trade on that exchange and this is not the case with RTB since ICO is still not over and distribution didn't take place yet ! So why they should make announcement about that !
Because this can attract more investors, if they know the token is already in IDEX listing visible. How many ICOs did you see, where the investors screams unpatient in telegram channels, in Ann Threads and in social media ; When Exchange ? When Moon? When Lambooo ? It would give another kick for to reach the hard cap may be
First, as a said, " already get listed " is wrong word since it's not available to trade. and IDEX is just decentralized exchange like etherdelta, getting listed your token is easy, all you have to is asking them to add your contract address, So it's not kinda news that will attract any investors or make them reach the hard cap !.
Well, some tokens have really good volume here.
Yes it can be that getting listed in Idex exchange is easy, but it also take some time after you apply for it. And most tokens get listed first in low volume exchanges; the investors know about it and I dont think they are gonna avoid the trading with RTB in Idex just because its a low volume exchange and easy to get listed in. But now after the ICO, they do not need to wait for trading, it will be proceed more faster. I think that giving this information to potantial investors will make a positive acceleration.