Payload used to have more maps but they lost popularity (as in the server stayed empty / had few people playing when the map was in play) so Bottiger had no choice but to remove some of them from rotation.
Bottiger also looks at TF2 wide map statistics. He can see how much or how little a map is being played across the game. Unless the removed maps gain popularity from a statistical standpoint you're probably not going to see the removed maps coming back or else the server would be at risk of losing players.
If Valve makes new payload maps that are popular you'll probably see them on Skial, but so far only borneo has taken popularity