Etherium is a better coin since it also has the smart contract feature. There is nothing that you can do with bitcoin but you cant with etherium. So why not all people sell their bitcoin and buy etherium.
People will not sell their bitcoin for ethereum. Ethereum has been hacked so many times, just as of September 2017 there more than your fingers can count hacking incidents that involves ethereum. That is so bad as for their reputation, that is why people hesitates to put all their money there. You wouldn't want to put all your money in it with that kinda reputation. So bitcoin is still most reliable in terms of invesment and security. Im not trying to devalue eth, because I'm an eth investor too Im just being fair on my judgement. Etherium may have a very big potential but the security hinders it to surpass bitcoin.