I'm not one to declare other people's motivation, but it strikes me as very odd that there isn't a "So, here's what's irreparably wrong with dpkg/apt and rpm/yum".
it's fixing a (real or perceived) problem with nix, not with dpkg or rpm.
Well, there isn't a "So, here's what's wrong with nix" either.
there kind of is, if you read between the lines - their usp is the scheme interface, which provides a "proper" programming language with which to interact with the packaging system. they don't explicitly call that out as something wrong with nix, but it's clearly what they feel they do better.
I support what they are doing. Wheels sometimes get better when they are reinvented.