Many ICO launch payment gateway choice ETH. any other crypto coins why not use in ico payment gateway.
Perhaps everything is because creating a contract in Ethereum is a lot easier than another platform, and also Ethereum is a well known Coins that many people has the hang of it in their wallet.
I agree with you, there are many ICOs projects be built on ethereum blockchain because it is first platform which offered to be used for smart contract and decentralize application dapps and the other reason is capitalization of marketcap ethereum is big, but the problem on ethereum blockchain is slow transaction processing and today there are many ICOs projects switch from ethereum blockchain to EOS blockchain because EOS is faster than ethereum we can check on here
https://blocktivity.info/The first reason should it was a popular platform and second will be ethereum used simple code that even the beginned can learn it easily. It was available in various programming languague. There more other reason too.