"Everything not forbidden is compulsory." – Murray Gell-Mann (from T.H. White)

Suppose we grant this claim as an axiom. Then, from the fact that X happened, we may deduce that X was compulsory.

But that doesn't tell us why X was compulsory. It doesn't provide us with an argument showing how the happening of X was a compulsory (or even probable) consequence of self-evident premises. Gell-Mann's axiom doesn't tell us why X had to happen, or even just why X happened — never mind the "had to". So it doesn't answer the question "Why does something exist instead of nothing?".