Listing on the exchanges adds liquidity and when the market turns around (no matter a month or a year later), one IDEX will not be enough to reveal the potential of the project. Or do you think that the volume of trading in the 15th ether is an excellent indicator?
IDEX is an independent exchange not affiliated with the project and the prices you are seeing on DEXes are not the true value of the tokens, the price through the GAMMA sale is what the team value the tokens to be at this time. The team will list the token on an official exchange once they feel that the underlying technology that the network will be be based on (both hardware and software) is at a level that is representative of the team's goals and vision for the project. They are currently developing that technology which is why they have chosen not to list the token on major exchanges at this time.
Please also refer to
https://app.xyo.network/gamma#col-full-113-119