Prices being too high to begin with.
It doesn't matter how useful crypto is (marginally, so far) or how much the underlying infrastructure has improved in the last year (a lot). If prices are rising at 10% per day for a couple months - which is what they were doing last December - then it's basically guaranteed that they're going to get ahead of fundamental value. Realistically, the only way out of this situation is for prices to fall by a lot, which is exactly what's been happening over the last year.
Bitcoin has the same price now that it had in August 2017. It's actually done about as well as the S&P 500 since June 2017, and a fair bit better since Trump's inauguration (it's up about 300% since January 2017, while the S&P 500 is up about 20%). It's just that in the middle of that time period it had this huge bubble, which just happens to be when most people heard about it and bought in, so the median crypto investor is now down about 80%.
