Prices are based on Bitcoin as a gold standard because Bitcoin is the first mainstream cryptocurrency.
Cryptocurrencies were designed as a unit of exchange and as a place to store assets without relying on a central bank. Bitcoin is often seen as the reserve currency of the cryptocurrency world. Rises and falls to the price of bitcoin often has a knock on effect with other cryptocurrencies. As bitcoin was the first mainstream cryptocurrency and is the most supported, the price of bitcoin can often influence the other cryptocurrencies.