Because not everyone realized the true potential of Bitcoin and the upcoming era of cryptocurrencies in financial world. Many people thought that bitcoin was a temporary thing, like they were thinking about computer in 90s and 2000s. Those, who saw that bitcoin went from some cents to ten dollars, thought it was the best time to make money from nothing and sold them immediately but there were people who knew that it was just the beginning and bitcoin would capture financial world. Yes, some waited and made millions of dollars from it, retired earlier but we can't deny that there are some people who wrote history by selling bitcoin for a few bucks and one of them is a man who bought pizza in exchange of 10000 bitcoins.