Because ethereum is in a field of its own when it comes down to token creation.

All the new coins provide the new solutions are actually still dependant on ethereum because none of them are capable of providing the insane level of protection ethereum provides. Ethereum gives you right to claim what is yours on an ICO, provides that security and there is no other coin like that which makes ethereum the second biggest coin in the market.

From now on, I doubt any other coin will be as big as ethereum for providing a platform for ICO's to give out their coins (creating a market for it) for getting paid in eth (getting investments) while the project is still underway (improving the coin) so it is only logical that ethereum is used in ICO.