Because the high sec groups were in need of a reason to contest sites. It was too easy to farm in highsec with 3 incursions.
The Null sec ones are utterly unrelated to the high sec ones. You aren’t going to get fleets contesting incursion sites in null, they will only ever be run by the home team, and I know for a fact at least 1 of the coalition’s has an incursion group. They just usually clear the incursions rather than farm them for a week to get rid of the cyno jamming. So they don’t look used.