Re: static class vs. static struct

[ref2401 via Digitalmars-d-learn]

For several times I've met struct(or static struct) usage in 
Phobos for singleton pattern implementation. Unfortunately now i 
can remember only core.runtime.Runtime.
So I've got a question. Why do Phobos guys use struct or static 
struct for or singleton pattern implementation? Why don't use 
static final class for this purpose?