Because the tippet has to be stiff enough to turn the fly over and to resist the twist an air-resistant fly puts into the leader. Two flies of the same hook size can present very different amounts of air resistance: a slim parachute and a foam-bodied pattern of the same size do not push the same way on the cast.

Tippet stiffness comes from diameter, not from the pound-test printed on the spool. A heavier tippet is a stiffer tippet: it carries energy to the fly, straightens the leader at the end of the cast, and holds its shape when the fly tries to spin. A light tippet under an air-resistant fly is pushed back by the fly’s own drag, and the leader lands in a coil.

So hook size is a starting point, not the only factor in choosing tippet. Look at the fly first: a large, bulky or foam-bodied pattern belongs one step heavier than a slim fly of the same hook size. The aim is not a stronger fight; it is a fly that turns over cleanly.

Source

From the Orvis tippet chart, on diameter, stiffness and air-resistant flies; see also leader and tippet; terms follow the FFI Fly Casting Definitions.