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
This is the pair that will continue to influence the value of the altcoin, and indeed Altcoin uses btc / usd in a benchmark price on ico or the market.
If btc rises and falls then Altcoin continues to follow it and this is also on demand. When looking at btc and green eth, other altcoins turn green except for altcoins which are adjusted to their communities.