At the moment, bitcoin is the most promoted of the cryptocurrencies. Most people only know bitcoin. However, the more people come into the cryptocurrency world, the more they learn about altcoins. The information that someone invested $50 in some altcoin and earned, for example, $1,000 also plays a significant role here. As a result, more and more attention is being paid to altcoins.
Bitcoin might be the most promoted or believed cryptocurrency but those who know about Bitcoin also know about altcoins, so that doesn't actually make them vulnerable to crashes. When Bitcoin's price drops, altcoins drop as well, why? Because people panic that they will crash badly and they start selling all their assets so that they don't need to face a lot of losses.
Altcoins are weak, and that is because people have more belief in Bitcoin and that is because Bitcoin has proved it in the past, whenever its price drops significantly, it eventually recovers, but when that happens to an altcoin, they hardly manage to gain any value back.