Because bitcoin is the first cryptocurrency that has been created.
I've read about the history of bitcoin hehe
Bitcoin started at the amount of approximately 1USD. The first transaction was that they used bitcoin in buying pizza which is they buy it by 10,000 BTC I think (Which is pretty cool. Because if you buy that btc in our present, you can now buy a house, a car, and a lot 
)
And because of that there are some people try to buy this btc and some investors are using it in investing (and because of that, the bitcoin amount started to increase.)
So today, the btc amount is now about 9k USD. And still hoping that it will increase to the next months
I've read about the history of bitcoin hehe
Bitcoin started at the amount of approximately 1USD. The first transaction was that they used bitcoin in buying pizza which is they buy it by 10,000 BTC I think (Which is pretty cool. Because if you buy that btc in our present, you can now buy a house, a car, and a lot )And because of that there are some people try to buy this btc and some investors are using it in investing (and because of that, the bitcoin amount started to increase.)
So today, the btc amount is now about 9k USD. And still hoping that it will increase to the next months