Bitcoin is gradually losing market value. At the start of this year, Bitcoin commanded about 70% of the total cryptocurrency market but with the continued surge of ethereum that has dropped to about 47%.
You seem to be talking about BTC dominance. Naturally, if BTC's dominance fell, then many actors moved to altcoins. This is the season the altcoin holders are waiting for. Regarding Ether, it is one of the top choices of traders. Why? I think Ether is the second alternative after BTC. Their community is huge. The developer is also always innovating. Their trust in Ether is very big, even though the price in USD may currently reach ATH, but in BTC the price of Ether is still very far from ATH.