Stackage has a much easier time since it only builds against one version of dependencies and one version of GHC. Adding a package to stackage also requires that someone makes sure that any external build dependencies are provided. Hackage on the other hand does not know any of these things so there is a lot that can go wrong.
But new library versions can also be temporarily blocked from stackage, so it can take months before a new version has docs on
stackage.org.
For library authors stackage and hackage, cabal-install and stack are to a large degree complimentary to each other, you will likely want to use both. Application authors have the freedom to choose.
- Adam