Almost any mangoes shelf life can be extended if its put in the fridge.
Keitt's reputation for long shelf life can be attributed to its ability to ripen properly from an early harvest.
some mango varieties if picked too early just do not seem to ripen properly, but Keitt is one of those dual purpose varieties, that can be picked as soon as the shoulders are full. and can take -3-4 weeks or maybe more at room temperature.
another factor is fungus on the skin, if recently picked mangoes are washed in some kind of anti fungal solution, this may prevent them rotting prematurely. anthracanose on the surface of the skin can spread rapidly.
I think its pretty safe to assume that any cultivar that has the ability to ripen properly if picked at the mature green stage, is a good selection for long shelf life.
Keitt is popular for exporters, because it can be picked green, and maintains a great percentage of its quality, when passing through post harvest processing, other attractive attributes, are size, and productivity of the fruit.
I have collected dozens of papers and reports on varieties with reported "long shelf lives" but this is all relative, a particular report may say that Neelam, has a good shelf life, but what are they comparing it to? Alphonso? Kesar? and how do those varieties compare to Keitt or KP. and what is considered good or excellent ? 2 weeks , 3, 4 ?
I get the feeling most back yard growers, prefer their fruit tree ripened, and I do not blame them, I also prefer them that way. or at least picked when just a few days before peak ripeness.
a big determining factor in the shelf life is ambient temperatures, the cooler it is the longer the fruit will keep.