Very basic stuff first. ICO works on the smart contract in correlation with the ethereum blockchain. They have easy implementation on that blockchain. As a part of this they have to maintain the standard of their token to a specific standard names as ERC20 which is relayed by ERC20 supported exchangers or wallets more often. To add this tokens the ERC20 protocol has to be merged with supported wallets and myetherwallet is one of them. That's why use myetherwallet. Oh yeah plus, it is most popular too. ;-)