it is simply because bitcoin is a well known currency with a lot of actual real world use cases, it is decentralized, secure and fast to use too. it is also immutable which is one of the most important factors in the blockchain technology. on top of that bitcoin has a limited and small supply.

while ethereum is a shitcoin that is centralized, slow, expensive and mutable. meanwhile ETH has unlimited and a gigantic supply that will never let its price go up. not to mention that ETH is practically useless. the only time you see it used is when someone wants to create a scam ICO token.