If you accidentally kick the plug out of the wall, the system returns to whatever state was last written down. To the program, no interruption is ever visible. The catch in an implicit system is that everything gets written down. For some special-purpose applications, transparent persistence isn't what you want. To ensure security, network connections must evaporate when the system restarts.
Databases require a means to write things down explicitly in order to support transactions.
An explicit persistence system is usually in the middle of an update when the system fails. As a result, the file system must be sanitized when the system restarts. Current file system implementations can take 30 minutes or more to recover on a large server after a crash. Until the file system is consistent, the system cannot let you run programs. While log-structured file system designs can recover more quickly, measurements by Ousterhout and Seltzer have shown that these designs don't work well under heavy load.