As of now BTC and ETH they are the market leader of crypto market. So everyone follows their pricing strategy. Altcoin believes their pricing strategy accepted by investor. Thats why altcoin follows them.
Ethereum is more focused on the development of the crypto industry, thanks to the availability and possibility of development and implementation. The bitcoin is geared more toward the ideal. Hence the impact on the market is obtained. Bitcoin affects everyone because everyone is leveled at it. And all, in turn, affect the Ethereum, because he bets on all the altcoins.