This should make a lot of sense, because we know that a hydrogen has 1 s as it's outer-most or valence orbital, so it can be filled up just with two 1 s electrons.

So, in this case, we're just drawing the molecular orbital diagram for the valence electrons, so we have three for each.

Valence shell orbital hybridization theory is exactly a synthetic theory that it has absorbed the advantages of these two theories.