Why is nearly every new coin based on ethereum?

Because creating your own "system & chain" would require you to also require you to get your own group of miners that are capable of and willing to mine said chain in order to make that currency work. Ethereum is a good system in regards to smart contract style chains, with faster transactions then BTC, although slower then ripple. But once again, it's more so on the fact that ethereum is specific for the smart contract system and speed to do so, where other coins are not at the same level.