Because of the empirical differences in the formal sources of the two systems.

Because both systems support that both international law and municipal law have a common underlying legal basis, which derives its origin from the law of nature and binds both States and individuals. 

Proponents of both systems regard law as a single unified field of knowledge.

Proponents of both systems emphasize the consensual nature
of international law

According to them, municipal law is logically a complete system.