That is because bitcoin is the entry and exit points, that causes the bitcoin to move less fractionally then other coins. Think about it this way, when someone wants to get out to cash fiat currency, most of the world still have local places that allows you to sell your bitcoin to local currency and send it to your bank account right ?

Well, that means if you have bitcoin you sell bitcoin, and that's it, however if you have litecoin then you sell your litecoin and buy bitcoin with it, sell your bitcoin and get fiat. Which means litecoin is losing value by getting sold whereas bitcoin stays the same because it is both bought and sold. That is why whenever people get out of crypto in general bitcoin still drops but drops a lot less then the altcoins that are getting dumped.