There is more demand for people/institutions who want to buy it than there is a supply of bitcoin from people who are willing to sell it. The buyers have to offer more and more money in order to convince someone who has some to sell it to them.
I don't follow bitcoin that close but I've heard there's been increasing interest recently from institutional investors. Elon Musk announced that Tesla had bought a bunch of bitcoin the other week. Since it trades secretly there is also always the possibility that it is being manipulated by institutional investors who are trying to create a bubble so they can cash out at a high price.
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