The badlands biome has the ruined portal type set to "Mountain". This matters because, as part of the mountain type, 50% of ruined portals spawn at level 70 and above. The basic badlands biome, the red sand desert, is not mountainous and will rarely allow for this type of generation. This means ruined portal generation will be greatly decreased in this biome.

Relevant pieces of code:

Badlands ruined portal definer:

this.addStructureStart(Feature.RUINED_PORTAL.configured(new RuinedPortalConfiguration(RuinedPortalFeature.Type.MOUNTAIN)));

Height definer for "in_mountains" portals. 50% of the portals with the "mountains" type use this:

else if (verticalPlacement == RuinedPortalPiece.VerticalPlacement.IN_MOUNTAIN) {
n4 = n - n2;
n5 = RuinedPortalFeature.getRandomWithinInterval(random, 70, n4);