because bitcoin is the only crypto with a proven track record of not being a shitcoin. no altcoin can say the same. hardly any other cryptos have been around enough to be useful, have supporting infrastructure, or a dedicated user base. while some alts may prove they are not shitcoins in the future, you must always assume they are until they have been proven otherwise. since bitcoin is 48% of the market cap, when it's price dips, all shitcoins dependent on it, also dip, and they dip a lot more, based on how speculative they are.