The biggest reason is because Bitcoin was the first and because of that bitcoin is the pioneer from any other alts and most of the another currency is just copycat of bitcoin and one more thing is Bitcoin has the most secure network. So i think this is the reason why when bitcoin fall any other alts will fall too.
You could say bitcoin is the father and mother of cryptocurrency. it is natural for altcoins to follow the development and market growth of bitcoin.
no one would have thought that there would be an altcoin that could replace bitcoin's position in the crypto market. it's too vital I guess.