In Java, a static initializer can run code during the initial loading of a class and it guarantees that this code will only run once. Also the static code will finish running before a class can be accessed in any way.

Initializing static members from constructors is more work. You have to make sure that every constructor does this. You need to maintain a flag to mark the static work when it is done. You may have to think about synchronization or races conditions for work in static block not initialized from static context.