The incremental, attrition-driven pace of PL is great for a gamemode involving lots of players on a wide range of classes, but 6s is best suited to zone-control modes on maps with distinct areas and connectors, which 5CP is perfect for. KOTH has elements of both styles, so it ends up working for both gamemodes (low map overlap obviously due to player count difference)
A/D does fundamentally suit 6s gameplay, but the asymmetric dynamic does not. The only reason anyone tolerates having to spend half of every game holding points is the ability to re-push or countersack after a successful defense. I also think the mental disconnect created by the asynchronous scoring system would be a bigger problem for 6s players than it has been for HL players, and would be a pain to get people on board with
I don't know if there's any 6s-viable A/D maps out there anymore but I do think a weekend A/D only cup could be fun, but I think it would be more worthwhile to experiment with mapmaking for more minor variations, like 7CP, multi-KOTH, or whatever Standin's mode is called