It is because Bitcoin is the most popular cryptocurrency. In fact, many people still don't know other cryptocurrencies exist. They first only know there is Bitcoin which is magic internet money to them. Second, they know it is a cryptocurrency. And the awareness stops there. So traders will always flock to the safest coin in bad times.
I agree with this. It's a fact that when we say cryptocurrency to them, they only know that it's none other than bitcoin. And another thing, if we say blockchain, they'll only think about bitcoin as well.
Wise investors like to get into a safe haven and bitcoin is the proper coin for that.