For vgo there is an explicit goal is to avoid NP-completeness for some reason. Systems of not-equals are NP-hard (trivially models 3-colouring), so allowing not-equals constraints on package versions is a non-starter if the stated goals of vgo are to be maintained.

Personally I think that some kind !=-requirements are needed for package managers just for this case, and thus the choice of dependency resolution approach vgo has is wrong IMHO.