if I think it might be
1. people are tired of Bitcoin which has always been the top and hopes there is a coin that can replace Bitcoin, and people want Ethereum to overcome Bitcoin.
2. according to people maybe what is worthy of being the top rank of cryptocurrency is Ethereum and not Bitcoin.
3. maybe people feel that the project from Ethereum is better than Bitcoin.
It depends on what side is using the coins more....Ethereum is not Bitcoin and there is one side that wants smart contracts -the other one wants regular transactions.