Thanks for the responses. The picture is becoming clearer. Seems to be a number of factors:
1. Aircraft structural considerations
2. Aircraft operational considerations
3. Aircraft performance considerations
4. Operator financial considerations
You can increase MLW (up to a point) if you're willing to accept lower service life and/or increased maintenance actions. Perhaps an example will help.
The C-17 is designed for a 30,000 hr service life. That service life assumes a "load spectrum" of loads imposed by:
flt hours at cruise altitude
Flt hours at high speed cruise at low altitude (below 5000 ft)
number of pressurization cycles
number of touch and go landings
number of full stop landings
number of assault landings
number of rough airfield landings
and many more.
The actual load spectrum the C-17 sees is different than that assumed during design, flying many more hours at low altitude and making many more assault landings and landings into rough unpaved fields. To achieve the design life, the maintenance program has been modified (for the entire fleet) to account for the harder than planned for use. This benefits the RAF C-17s which don't do many (any?) tactical operations into unpaved fields, But that "benefit" also translates into higher than necessary maintenance costs. Interestingly, the C-17 fleet is now being maintained with an eye to achieving at least 3 design lifetimes. To put that into perspective, at current flight hour/utilization rates, this means that just over half the fleet will still be flying in 2117, just over a century from now!
BTW, C-17 MLW is the same as its MTOGW. But this is on paved fields and "normal" landings (2.5 degree glideslope with a flare before touchdown) When doing an assault landing (5 degree glideslope and no flare at touchdown) the MLW is (predictably) reduced. This limit is (primarily) driven by fuselage bending moment. Besides the gross weight limit of the aircraft at touchdown during an assault landing, there are limits on the amount of fuel allowed in the outer wing tanks. This is mostly driven by wing down bending moment. Since normal landings are much much less stressful than assault landings and the structure is designed for the worst case assault landing (into a short, rough, unpaved field) the structure can easily handle the normal landing at MLW. (In other words, no need to dump fuel if returning to base right after a heavy takeoff.) It also means that the structure does not fatigue at its design rate if the airframe only experiences "normal" landings. Which MIGHT mean reduced maintenance actions. But it comes at the price of an "overbuilt" (and therefore heavy) structure and landing gear components.
An airplane MAY be designed for a specific service life assuming a certain number of MTOGW takeoffs followed by a certain number of flight hours and a certain number of pressurization cycles with no specific MLW. Once that structure is designed, the MLW for that structure can be back calculated and that becomes the certified and/or warranted MLW.
I hope this made sense and was useful.