The price and, ultimately, the charts on any given exchange are determined by the price of the most recent transaction. How can the prices of 100s of cryptos all move in the same direction at the same time?
They adhere to a general trend, with Bitcoin remaining the one that they largely follow because it is still the best cryptocurrency to use. Additionally, the general mood of one sector of the market typically affects the other sectors, thus when Bitcoin is down, it certainly raises a negative mood (for the most part) in the altcoin sector. Not to suggest that altcoins don't have their own market; pumps and dumps still differ from coin to coin, but most of the time whales are controlling how much a market pumps or dumps.
It is understandable because people get into crypto world via bitcoin mostly, and then they start to look at other things as well. Not all people but most people buy bitcoin as their first coin, and then they could end up buying other things for bigger returns if they get greedy, or they could be like me where they end up with just bitcoin and maybe a bit of eth as well.
It is clear to me that it is the best option, but also more than that we have a HUGE futures market as well, people long or short bitcoin a lot too, and that is even bigger peoples market, billions change hand there everyday which causes the market to be huge and bitcoin being bigger than all others.