Because by cooperating with an Exchange it is certain that the coins can be traded (can be sold in the market). it's no secret that people who make investments, campaigns, developers, they definitely choose coins that can generate profits, not junk coins that only settle in the wallet.
There was a basic knowledge of investors and when the coin can be traded on the exchange site instantly and then it can try to join into the hype and that will make the price of coin go to the moon.
This is a common knowledge that has already known by almost all of people.