Why is creating a bitcoin fork very difficult and a Ethereum fork easy? There are a dozen of projects based on Ethereum but very few based on the Bitcoin network. Why is this?
Forks of both are equally hard. What you're describing as "Ethereum forks" are actually tokens/smart-contracts that run on the Ethereum network, which isn't very common on Bitcoin (but there's Omni)

EDIT: Remved Rootstock as per darosior's clarification. Its only to relation to Bitcoin is that it uses merged mining, just like Namecoin.