As according to my point of view the reason behind to follow the BTC by altcoins is that the BTC is the first cryptocurrency which comes into the existence.
And the holders have more trust on it as compared to the Altcoins and BTC is the market holder it has the power to fluctuates the market.
Bitcoin is actually the back bone of crypto market and it makes a perception in the market which his not just limited to a single cryptocurrency but to all the crypto market. When we see improvement is the market of single currency we have a good perception about the crypto market and that is why success of any cryptocurrency generally is a growth for the market.