Because our society is stuck on a finite planet, using a debt-based monetary system in which counter-party obligations expand exponentially. Without an alternative monetary system, the disruptions which occur when the exponential growth stops may be of a scale on par with an extinction level event, in the most extreme scenarios.

All of the transparent block chains create another problem, however: Pervasive surveillance, and chilling effects. For that reason, I like the singular cryptocurrency which offers the anonymity and fungibility of cash, without a pre-mine scam or a graft tax, Monero. I think it is the only exemplar which offers all of the features required to serve as both a store of wealth and a medium of exchange. (Privacy being a requirement for fungibility.)