People didn't believe in bitcoin, so they sold it in 2011, 2014, 2017, and so on.
It is not always like this. People have many reasons to sell bitcoin like they think it's good time to sell it, mandatory and unavoiable selling demand by having urgent need of money, want to outsmart the market by timing the market, finding absolute chance to take profit at ATH and wait for bottom price to reenter, and many things like using leverages, futures trading for possibly better profit and get liquidated eventually.
The future is not a guarantee so it's all in our decision whether you want to take the risk or sell and back to the old centralized banking system for your payments.
First ever crypto is not a solid reason for someone to hold on to it but it's the decentralization part that can convince them to do so.
Future is uncertain for everyone and it's easier by looking backwards and say something but future is hard to predict. You must have very good strategy for your financial management, and make sure that your life won't be broken if something very bad happens with Bitcoin.