It's simply Bitcoin is open source meaning people could read, improve, develop their own coin but can't alter the functionality of the existing coin. Now since most coins are built in this software prices of Bitcoin tends to affect them too. What ever direction Bitcoin posses ather coin know as altcoin takes up the same direction. And also Bitcoin happen to be the first every cryptographic project that ever survived due to its decentralized nature and as such it is the main target of the market and what ever effect it has on the market inveriably affects other coins