Quite simply because it went up in price at an amazing rate, and when it fell sharply, as it often did, it quickly recovered. If you had bought $100 worth in July 2010, when they were 8c each, you could have sold them for $237,500 in December 2017. It would quite commonly increase by 10% per day, often more, and there were many points (even two weeks before the crash) when you could double your money in a week or two.

However, in December 2017, tradable Bitcoin futures contracts were introduced, making it easy to take short positions (profit from it falling). In the following six weeks, the price fell by 70% (from $20,000 to $6,000). Since then the price has been much less volatile, and presumably will remain so. Although $6,000 appears to be a floor (for now, and linked to the cost of mining new coins), the volatility (change in price over time) is constantly falling. This chart shows the Average True Range (average movement over the last 14 days) constantly falling since the peak.

Cryptocurrency trading are still interesting to trained market traders, but the one-way bet attraction for amateurs has now gone.
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