The difference is in the way they are adopted. Bitcoin is individuals using & holding BTC, Ethereum adoption is about companies building DApps into the ecosystem, which is currently limited by the scaling etc of Ethereum. In order for Ethereum to get a higher market cap than Bitcoin it first has to fix the scalability issues it keeps hitting & the changing fees. Without that, companies will turn to alternatives or create their own code-base forks of the protocol to just run their services.