For approximately the same reason it's more dangerous to climb on the roof of a hundred-story building than a one-story building: there's much farther to fall.

I also think its price has become thoroughly decoupled from actual use value. Back in the day one could plausibly (if optimistically) argue that it was going to become the new cash, the new credit/debit transaction. But it has obviously failed at being the digital cash it set out to be. (With the exception of the untraceable envelope/briefcase of cash popular with criminals of various sorts.) There's also now a much richer market manipulation apparatus. That increases risk directly, but also indirectly via increased regulatory interest. Tether, for example, is under heavy investigation.