Upvote:13
It is easy to view the distance between Botany Bay and Port Jackson as small by today's standards, but it is actually a considerable 'trek'. Even with todays excellent roads, it takes two and a half hours to walk from Circular Quay (and the better soil around Port Jackson) to the northern edge of Botany Bay.
Then consider you have to carry supplies over multiple trips and this short distance becomes a great deal longer.
But the most important point is indicated in the Wikipedia article:
excellent natural harbour of Port Jackson to the north
A deep, well protected harbor was incredibly valuable to a new colony that would rely on support and contact with Britain to survive and, in the long term, trade to prosper. I imagine that the proximity of such a harbor makes Phillip's decision even easier.