Bitcoin was developed as a store of value. It is meant first and foremost to be used as a currency, and that is what it is good at.
People know Bitcoin to be the definition of a blockchain (as correct or incorrect as that maybe), and when they go to invest in cryptocurrencies, they think about Bitcoin first.
Bitcoin has the purpose, and Bitcoin has the branding.
Ethereum was developed as a platform for “smart contracts”. The currency, Ether, exists as a way to make transactions on the Ethereum blockchain, but the blockchain’s focus is on being a place for dapps to exist.
People are less familiar with Ethereum. They only know it is Bitcoin’s younger, less valuable brother.
Ethereum doesn’t focus on value in currency, it focuses on providing a platform for others. On top of that, it is less well known by the general public.