Sort:  

It should not be pegged. But in reality it is because BTC is 50% of the wealth of cryptoholders. Cryptoholders in general are people who believe in the technology and have some money.

People who do not believe in this or do not have money stay outside (worldwide these are 99.9% of the people).

So when cryptoholders/believers get decimated, all coins go down, as there is less money/wealth available in the community.

Here another reason why. Bitcoin can take the damage of banks bc of its large network and brand name. Bitcoin can be the main way to use fiat and other trading pairs take advantage of that. Do you really think altcoin even really popular want to risk it? Another reason is so that every crypto is related to each other so they don't have to fight head to get listed their is one common pair they know they will get.