A major factor here is that, especially historically, the vast majority of trading volume for coins apart from BTC/ETH is priced in either of those two. Unless

A major factor here is that, especially historically, the vast majority of trading volume for coins apart from BTC/ETH is priced in either of those two. Unless you have aggressive arbitrators and liquid markets with thick and deep order books, this means that smaller-cap coins end up tracking the major ones. It is only relatively recently that ETH significantly decoupled from BTC in this sense. There are also infinite ways to construct and weight a price index. Go to different websites and you will notice significant discrepancies. One doesn't necessarily have to be more "accurate" or "better" than the other. Right now everything seems to be tracking the NASDAQ, though...