ETH platform offers easy to set up Smart contracts which can be sued to make the project over the ETH blockchain. This way these projects doesn't need any sort of blockchain of their own and thus they can get started quickly. This smart contract address also offers them easy to transact features where at the time of ICO people can just send the ETH to contract address and thus get their tokens instantly into their ERC 20 supported wallet address. This process is far simpler over the ETH platform as compared to the others and thats why they are using the most. Plus, ETH is most popular currency to be used for such transactions.