I believe the short-and-sweet explanation is that the Haftorah of Acharei includes the gist of the Haftorah of Kedoshim.
Thank you Fred, for pointing to the Mordechai, Megillah 831 as well as the Mishna Brurah 428.26.
The Haftorah for Kedoshim refers to "the sinning city", and is just a litany of its faults.
We basically make that a closet Haftorah as much as possible. Decry certain practices, yes. But a Haftorah that just calls Jerusalem a sinning city? We don't like to do that out loud very often.
If we read Machar Chodesh for Acharei, the Mishna Brurah says we ignore the "Kedoshim haftorah" and read the "Acharei Haftorah" on Kedoshim. Hooray! Another opportunity to skip the "Kedoshim Haftorah."
I'd seen a sermon online (by a rabbi who's sadly no longer in good standing with the law) about this, with implications for current events. If I recall correctly, in the very rare event that you read the Acharei Haftorah on Acharei, and there's no special Haftorah to read the following week, then while the Mishna Brurah says well, now we're forced to read "the Kedoshim Haftorah", in fact there are Briskers who will read "the Acharei Haftorah" a second week in a row. I.e. it's truly a closet Haftorah.