Bitcoin is a cryptocurrency used by both individual and investors both in trading.
It is not only for trading.
The main usage of bitcoin is as a currency and many people (even countries) are using it as a means of payment.
Due to its pioneer status, a lot of investors views it as the crypto market reserved so another coins depend on its value to remain high.
That's true that many think this way but it is not a correct assumption.
The fact is that bitcoin was not the first attempt. There were others before Bitcoin that failed. Bitcoin was successful and is growing today because it works as it is supposed to work.
Alternatively the altcoins all failed and will continue to fail simply because they don't work as they are supposed to and they have no real utilities to offer!
Why Bitcoin price change, impact other coin's.
To answer your title:
The reason is simply because altcoins are useless and traders know this fact!
That means whenever bitcoin price goes up or down, they immediately pull their money out of the altcoin market (ergo causing altcoin crash) to put it in bitcoin that is the only thing with potential.