Both of these coins (BTC and eth) are very influential on the value of other altcoins, and exchanging all the coins can go through these two coins. So the ups and downs of these two coins are very related. Btc and eth will be top coins and have developments that will continue to improve.
Its true, most pairs on exchanger using bitcoin and ethereum as main pairs. Most altcoin price depending on BTC/USD and when the price that pairs increase, most altcoin price increasing too