Mostly for game balance reasons and the limited granularity of Shadowrun's damage system. Just how damaging do you need a knife to be, compared to a two-handed sword, for instance? And just how over-the-top silly would the same weapon be if you did put the same upgrades on a katana or a claymore?
Maybe, in-universe, there's just a limit to how danged sharp something can be, and (mechanically) how effective it can be at cutting through stuff. Once something is already as wicked sharp as a mono-knife, it just stops getting much better at its job, and more and more its combat effectiveness comes down to the wielder, not the blade.
