Well, because of bosses and big foes. You can backstab bosses and some big enemies at all, so all your time and resources spent into a backstab based build are lost.
After you carefully planned a backstab based build, you realise it’s trash against bosses and creatures you just aren’t allowed to backstab.
So, they should balance that in order to make these builds suitable somehow. Maybe you could do some minor backstab or something alike with minor damage, otherwise it’s just a bad build.
The “on kill” effect has the same problem against a boss In my Enchantments Rework I Fixed the problem by grouping Stagger, Backstab and Kill in a single keyword: “Dominance”.
if you want to take a look here it is: