The thing is, on the outside the 737 MAX 8 and the MAX 8-200 are visually identical, apart from the extra emergency exit. However, they were able to get around this by “simply” making that extra emergency exit a part of the liveries that have it (I.E. Ryanair, Malta Air, etc.). Instead of it being an actually modelled door, it’s really just paint.
The A321neo and A321LR are visually identical, as far as I know (anyone feel free to correct me on that if not). However, the A321XLR does have two external visual differences compared to it’s sisters.
1.: Unlike the inner flaps on the A321neo and LR, the inner flaps on the A321XLR are single-slotted, as opposed to double-slotted like they are on the other two.
2.: On the A321XLR, the fairing is noticeably extended at the rear (for extra fuel capacity).
Now, while these might be “small” things, this does make it more complicated than the MAX 8 - MAX 8-200 situation.
To be fair, they were able to make significant changes to other models in the past, such as the 747-8 (a frankenstein freighter version), the Q400 (a special firefighting version) and the DC-10 (an even more special firefighting version, as well as two tanker versions). And that’s not to mention the aircraft with smaller changes to the model, such as a different engine type, different wingtip device, different armament configurations, or even the C172, which has not only two different cockpits, but also two different landing gear types - one with an aerodynamic cover over the wheels and one without it. So, I don’t see what’s so different in the case of the A321neo in that regard.