• Singletons are objects and it might be more intuitive to deal with them for state manipulation rather than through the use of static fields in static classes.
• In the same line, you can't pass a static class as parameter, while no such limitation exists for the Singleton object.
• Static classes cannot implement interfaces. The class that defines the Singleton can.
• The global static class can be loaded too early, consuming valuable application resources, while the Singleton supports lazy allocation where an instance is created only as needed.
• Static classes do not benefit from default serialization mechanisms.

BR,
-- AFurtado