IME the #1 cause of cracked chromoplastic mudguards is that they were not fitted correctly. Some top tips;
1) make sure the stays are secure and that they are not strained sideways when fitted; in other words they should be 'set' so that they sit in the correct place naturally, even when not attached to the mudguard
2) avoid straining the mudguard curvature esp near the brake bridge. If necessary space the mudguard away from the chainstay bridge so that there isn't permanent strain on the mudguard.
3) chamfer/trim the mudguard if necessary so that it fits between the chainstays without being strained.
4) if some strain is unavoidable, you can locally heat the mudguard (eg using a hairdryer) so that it takes a 'set'. It is less likely to crack vs being strained when fitted.
5) fit the (stainless steel, the plastic ones are crap) brake bridge mount so that the bottom of the slot is not exposed, if necessary by using washers either side of it,
6) crimp the bridge support so that the mudguard doesn't rattle there
7) avoid having the stays differently bent one side of the bike to the other; this is commonly the case on disc brake bikes but it encourages the mudguard to flap from side to side more than it should.
make absolutely sure that your bike is in track; if it is slightly lopsided (eg a pannier on one side is enough) then again the mudguard will vibrate from side to side more and this (other than massive strain) is what destroys them most quickly.
9) use nylocks etc so that the mudguards are not likely to work loose
Basically if you slacken any one mounting bolt and the mudguard immediately moves under its own steam, there is some strain in it somewhere, and probably this can be reduced or eliminated, thus giving the mudguard an easier time of it.
I agree that the rear light is best put somewhere else; a lightweight reflector is much less likely to strain the mudguard. Having broken a couple of mudguards with the light on (they seem to crack at the middle stay mounts when the light is attached, and strain all the other mountings more too ) I have my light on the seat stays.
I think that these mudguards are polycarbonate and therefore eventually embrittle and become likely to crack. Some solvents and UV light will speed this process up. But I have had a set of chromoplastic mudguards last about 30 years (mostly outdoors) before I broke them; however when they broke they were as brittle as glass.
I'd also note that skinny mudguards and skinny high pressure tyres are a bit of a double-whammy; the mudguards see a lot more vibration damage with this kind of setup.
cheers