Because when the value of Bitcoin increases, it will for sure also pull the top altcoins in the market for sure, this has happened several times so when the bull happens, for sure most of the altcoins that have real potential will increase.
How do you know then which of the altcoins has real potential and that's why investing in altcoins is very risky because we don't know which one will make it. Not all altcoins rise with Bitcoin when it rises, some also falls as traders dump them to buy Bitcoin. If you're unfortunate and buys those that'll be getting dumped like the memecoins, you'll lose. We don't have to limit our investment but when investing in altcoins we have to be extra cautious.
It's better you avoid investing in altcoins totally because they can make you to lose money more than making you money, altcoins rise fast and give huge returns than Bitcoin but they also dump faster because many of the altcoins are just used for pump and dumps by whales in the market.