Could they have done that? Almost certainly.
Should they have done that? Only if their goal was to create a toy that was as accurate as
possible to the filming prop.
I submit that Nerf's actual goal was to allow 12 to 30 year old to pretend they were Mando whilst shooting their friends, classmates, and co-workers with bits of brightly coloured rubber and foam. To me, that suggests the design targets were to create a toy that is as accurate to the filming prop as
practical while retaining acceptable range and accuracy of the dart, and without exceeding limits of manufacturing cost, shipping cost, and safety regulations. Evidently that couldn't be done without shrinking the length of the toy.
Since the EE-3 is shorter (and much smaller proportion of its length is barrel, thank you
benhs1898 for pointing out the mechanical issue) making a working dart gun within financial and regulatory limits will probably require a lot less compromises to the screen accuracy. That's just my opinion, of course! Unless Hasbro decides to release sales targets and results on the Amban, we'll never know if it undersold.
Having stated that opinion, let me acknowledge that the Amban does still seem to be fairly available in stores (at least in Canada), the reviews are low, and that complaints are fairly evenly split between "it doesn't look right" (though when they specified, the reviewers were more upset with the paint job than the proportions) and "it doesn't shoot well." I guess Nerf's attempt to split the difference between appearance and performance just resulted in no-one being happy.