Well, at that time, the vast majority of people didnt even really understand what Bitcoin was for. If someone took the time to study it and understood that its an alternative financial system, a way to preserve purchasing power, everyone would just hold... but no, people insist on treating it like an asset, some kind of company stock. So of course, when the price goes up, they get greedy and sell everything, without even realizing that Bitcoins natural path is to keep multiplying in fiat terms over time.
Because maybe they considered the use of bitcoin at that time as a fun coin, maybe he didn't read the whitepaper so he didn't know what the real use was in bitcoin.
At that time, maybe many people thought bitcoin was high - went up a little they immediately sold it and wouldn't buy, but over time they began to regret why they didn't hold it longer, but in this case I myself felt it in 2017 and sold bitcoin because it felt high.
And there's no need to regret it anymore, now there's still time to buy, it's never too late as long as bitcoin always records a high ATH record every cycle.