People didn't believe in bitcoin, so they sold it in 2011, 2014, 2017, and so on.
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.
Peoples were also still didn't believe in bitcoin because they were thinking there is not a guarantee for future and that's why they were also solding their bitcoin.
But we must know and keep in mind that investing on bitcoin will give you guarantee when you are holding bitcoin for long-term, bitcoin makes profited those investor who hold for long-term not those who get easily affected by the fud and sell bitcoin.
So when you want a guarantee from Bitcoin you shouldn't make go for the short-term