Many ICO launch payment gateway choice ETH. any other crypto coins why not use in ico payment gateway.
They use ethereum because you can easily create a crowdsale contract, it literally takes you about 5 minutes to set up one or use a company that have this service and you can do this with a low fee...even if Eth had his ups and downs (look at cryptokitties fail) it is very cheap and easy to move tokens from one address to another.
ethereum platform provide developers team simpleness to create smartcontract.and also investors already accustomed with this platform.they could easily send their ethereum to contract and no need long time to recieve their tokens.