While before cryptography was, by and large, a purely computational and information-theoretic science, with strong guarantees built on security assumptions that are close to absolute, once money enters the picture the perfect world of mathematics must interact with a much more messy reality of human social structures, economic incentives, partial guarantees and known vulnerabilities that can only be mitigated, and not outright removed.
That says it better than I could. However I would also have said one of the underlying problems is evil, such as manifested in a 51% attack (that's about one specific attack, but it applies to cryptocurrency generally). Lots of interesting problems to solve!