Kardur sets up a game rule (essentially). Dress Down changes the characteristics of creatures. They have always been two different types of effects governed by two different sets of rules. Adding/Removing abilities or changing characteristics when done on a trigger or an instant or sorcery can only affect the subset of objects it wants to affect when the ability/spell resolves. Once the effect applies, it is done. It won't continue applying to new objects in that set.
So, Dress Down as an Instant can't work against something on the stack and can't shut down triggers at all. You can't set up a Continuous Effect on a resolving spell or ability that affects more than the objects in the current set because the rules specifically don't allow it.
It is the reason
Sudden Spoiling (which I believe was mentioned above) can't affect new creatures. it is the reason Craterhoof can't effect new creatures. It is the reason
Languish can't affect new creatures.
Kardur on the other hand just says something true about the game itself: that person's creatures must attack. It is not changing anything about the creature's themselves which is why, incidentally, Dress Down wouldn't change anything about the requirement. It is not a characteristic or ability of the creature itself. Similar to how
Rogue's Passage doesn't set a characteristic of the creature either. Which means Dress Down can't remove the Unblockability.
It would take a lot of rules text to make it work as an Instant/Sorcery to get around the current rules of resolving spells or abilities and changing the rules to suit it is out of the question. Otherwise other interactions, such as the ones above, get messed up. The way Sudden Spoiling works is how this would work if put onto an Instant. Putting it onto an enchantment offers the functionality Wizards wanted in a reasonably elegant way, if a little outside the norm.
On an added note, and I am sure this isn't really their intent, but Dress Down still lets things get their abilities back in the End Step whereas Sudden Spoiling wears off in the Cleanup Step. There aren't a lot of situations where this distinction matters but for something like
Jin-Gitaxias, Core Augur it does. Sudden Spoiling would still cause the person controlling Jin to discard to 7 cards. Dress Down does not. Again, a very niche interaction and one that I am guessing isn't really the reason for anything, but an interesting side note about the two different effects.