Why is HIV-1 global whilst HIV-2 is remains largely confined to West Africa?

HIV-1 is by far the more virulent of the two species of HIV.

Because HIV-1 is more easily transmitted, it also spreads more readily and therefore accounts for the vast majority of global HIV infections.

HIV-2, because it is much less transmittable, is largely confined to West Africa (where it is thought to have originated) and to West African migrant communities in Europe.

HIV-1 also mutates more efficiently that HIV-2 and generally progesses to AIDS at a significantly faster rate than HIV-2 does.