Why are hand-luggage allowances conditioned by airline, and not by airplane?

This is a good question but when you book a flight, you choose an airline, not a plane. Some people may be curious enough to look at what plane will fly the route but certainly not most people.

Most importantly it makes things less complicated. It is already difficult to deal with multiple different allowances by different airlines and trying to fit your gear so that it satisfies everyone's allowance is simply a pain. Imagine if you had to satisfy an even greater set of allowances due to different planes being used as part of your journey.

Honestly I wish they would all get together and settle on one size and let all the luggage makers know so that pieces meant for carry-on would be always right. There are too many variations now and we have carry-ons that fit one airline but not another, depending if they measure by dimension or linear size. Having to also consider the type of plane would make the carry-on allowance pages at each airline website complicated.

Some planes have less space than others but they rather the allowance be the same and sort it on boarding. Regional jets flying the US and Canada often have small overhead bins and the crew often passes around before check-in to collect larger pieces for gate check-in. This helps people not having to worry about the type of plane used and not consider two requirements if the return leg of the journey is on a different plane which recently happened to me going from YUL to YYZ. My plane going was a tiny propeller plane (first one I fly in my life between two cities - had no idea they still existed) and we were told to put the heavy carry-on under the seat and gate check-it if it did not fit. On the way back I was an a small but more common Boeing 737-800.