We have the Haskell Platform, which is basically a bunch of our "best of breed" packages we believe everyone should have installed by default. These packages are all maintained by individuals.
Mark Lentczner takes care of herding the cats to get that out the door.
GHC itself has a smaller collection of boot libraries it needs to build. Some of these are very GHC specific and fall under the purview of GHC HQ itself.
Some of these actually have dedicated maintainers, even though GHC needs them. e.g. Ross Paterson still owns transformers, he wrote it, its his to do with as he pleases. We do have to work with him fairly closely, however, as, being a boot package, it can only really effectively be upgraded every year or so, and we often have to maintain patched versions of it to keep developing on GHC in the meantime.
But, there is a fair bit ligaments and gristle between the bone of GHC itself and the body of the platform it, i.e. stuff that has to be there to hold the whole ecosystem together, but on which GHC HQ itself doesn't have a particularly strong opinion.
That is the stuff that falls under the purview of the core libraries committee, which acts as a collective maintainer under the libraries submission process.
That rounds out the idiosyncrasies of having a single maintainer, but lets us have some entity that can collectively reach a decision and allow progress.
Prior to the formation of the committee there were a number of issues on these middle-ground packages that would deadlock without universal consent.
The packages random, vector and primitive recently came under the purview of the committee because they were in the platform and seeing broad use but not being actively maintained.
By way of contrast, mtl isn't currently under the core-libraries-committee, but it is in the platform. I took over maintainership of it personally a year or two before we formed the committee and haven't yet found it to be a sufficient burden to pawn off on the collective.
-Edward