We have organizations chartered for land, air, and sea superiority. It seems fairly reasonable to have an organization chartered for space superiority, instead of leaving that adhoc across the other forces.
The navy was up there to maintain superiority over water. The airforce over the air. The army likewise.
But nobody clearly owned maintaining superiority over the domain of space and any operation that needed to ensure uncontested operation in space was a joint agency nightmare.
Space force is chartered to provide superiority in space so the other services can conduct their operations up there. Space is clearly a specialization that is differentiated from the air.