Why is creating a bitcoin fork very difficult and a Ethereum fork easy?
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.