This is because less of the surface of the fastener is cut away to make the thread. But you're not going to develop the full strength of the fastener until you have adequate thread engagement, which you're not going to get tapping into an aluminum mast or boom. With similar strength materials for the fastener and the female thread, you need about 1 1/2 diameters of engagement to get full strength of the fastener. With a steel fastener into aluminum, you need a lot more engagement.
Personally, I use coarse threads when I tap into aluminum.