Why reserve a strike carrier behind the fleet?

Would battleships be better than strike carriers for delivering the decisive attack?

No; the strike carrier could stay concealed behind the fleet and deliver a timed mass air attack while battle carriers supported the main force.

The strike carrier was intended to remain harder to find behind the fleet. At the chosen moment, it could launch the concentrated blow that American and Japanese doctrine also sought. Battle carriers would protect and support the main force in the meantime. Plans considered Japan the most likely opponent but also examined the United States as another major naval power.