Simply because it has demand on it. Bitcoin is the first cryptocurrency, lots of cryptos are being traded through Bitcoin, thus there is a high demand on it and for a good amount of time it has an over 50% dominance. It's also because during bear markets Bitcoin usually holds better its price than altcoins. However, when altcoins start to rally again big amounts of money are "thrown" into such cryptos and Bitcoin loses its dominance. I think that this might be the last year in which Bitcoin will have that much of a dominance. In the next three years I think other crypto projects with much more use cases than Bitcoin will take away its huge dominance.