Why is Equiano allowed to stay on the deck rather than below?

Equiano was kept on deck because of his propensity to be sick.

Happily perhaps for myself I was soon reduced so low here that it was thought necessary to keep me almost always on deck; and from my extreme youth I was not put in fetters. In this situation I expected every hour to share the fate of my companions, some of whom were almost daily brought upon deck at the point of death, which I began to hope would soon put an end to my miseries.