Who is actually spending their bitcoin on eth when you can hold btc instead? Are people currently buying straight into it from fiat or are they giving up precious btc in order to board the eth train? At this point in time i wouldnt dare move my bitcoin into something like that.
It seems like a lot of the volume is traders looking to hedge some Bitcoin positions. Volume is fairly low for Eth though - which is very different from their previous run ups where we saw big spikes in ETH/BTC volume. Mostly it is just that Eth holders are not selling at all while Bitcoin runs up, so they are both going up in terms of fiat.
Honestly, it is the macro economic situation that is the main driver. People are nervous about holding fiat (USD, EUR and CNY all have major issues). Gold, bitcoin and eth are all looking like good bets in comparison.