The cryptocurrency rout that began early this year with the popping of the bitcoin bubble has only gotten worse, and there is little sign things will get better soon.

The total value of all cryptocurrencies fell below $200 billion last weekend, down 76% from an all-time high of $832 billion in January. It is a dramatic decline for a sector that experienced furious growth in 2017, driven by a surge in speculative interest around bitcoin.

The...