I think because eth holders are switching to holding bitcoin now. that made a decrease in the volume that occurred on eth which resulted in declining eth prices this year.
ETH/USD is totally in a downtrend (just like most other coins)
And another important point here is that ETH/BTC also in a downtrend => Even BTC increases (BTC/USD) fast, ETH (ETH/USD) only increase much slower.
Many bad news about ETH may be released in the near future to sink the price of ETH