It depends entirely on the texturing & modeling system they use for items. To dye stuff, you have to have your engine set up for it - you need to have a default color for use in textures that is replaced with a "dye" color. If you have a system that just uses straight texture / alpha / reflection / bump maps, it's hard to just say "ok, and now we'll make the colors in those textures alterable!"
(I wonder... did anyone ever make a dye mod for OB? "Dye" and "Dying" only return two search results on the Nexus, and those are both for the "Undying" race
)
...Oblivion's clothing texture system was a bit clunky. Each texture had to be assigned to an individual .nif (model/mesh), so you'd download a "40 different minidresses!" mod, and it would be hundreds of meg, because it needed 40 copies of the dress model, to go with the 40 different textures. Fallout 3 improved that a bit, letting you have one .nif and several different colors. Saved a heck of alot of space when modders wanted to put out collections of thirty different T-shirts, or bikinis, or ball gowns.