Why is bitcoin becoming more expensive and people are buying it instead of altcoins?

I'll give you a very simple answer to your question.

If you knew something is performing well and everybody is talking about it, even presidents, would you ignore it and buy something that's under performing and most people have not heard about?
Let's take ETH. It used to be much more popular in 2021 and worth more than it is today. Why would I choose ETH over BTC knowing that?

If I had ETH and BTC and saw ETH being worth less than it was 4 years ago, while BTC has reached a new ATH, I'd sell my ETH and buy more BTC and that's what people are doing and that's why bitcoin is going up and many altcoins still struggle below their 2021 highs.