There is a reason most 121 aircraft are certified to a max of 41k, and fly in the mid 30s (+/- 10k).
Ummm....it has to do mostly with PSID limitations,
specific to each airplane's limitations....yes?
In terms of MAX altitude. Along with weights and performance issues.
Example...the "classic" B-737 compared to the "NG" B-737. Different 'MAX' Altitude restrictions, based on changes in the construction OF the airframe. Also, engine changes....but the "Classic" B-737 was primarily limited to a lower MAX PSID because of its structural design.
The B-727? (Even though the same basic cross-section in the pressurized portion of the fuselage?)... Able to go up to a higher PSID (ETA...just realized I have to add this explanation for "PSID"....AND I am sorry, having trouble finding an ON-LINE source to explain the aviation concept of 'PSID". It means "PSI-Differential". It is the relationship to the internal pressure in the airplane cabin, to the external "ambient" atmospheric pressure. : ....the "concept" of the early B-737 initially was to go "lighter-weight", and as a "twin-jet" for short-haul applications, this made sense....back then....SO, the fuselages were built to withstand a LOWER PSID, in order to save weight in construction.
LATER, the B-737 NGs were "built-up" to be more "beefy" to allow higher altitudes, and keep PSID levels at a point where the INTERNAL cabin pressure was still comfortable for the occupants. New engine technologies helped, too.