Updates needed to fix e.g. security issues, which otherwise might not be backported if others are staying close to current. This is why Stackage has both LTS and Nightly; LTS only works if there's a *commitment* to it, at the level of the community for a community resource or at the level of the provider for something like ghc or Stackage.

Note that GHC HQ's response was that they have had problems finding people to keep multiple versions active at the same time; it's a significant job given that backporting (say) a fix to a type system issue allowing unexpectedly unsafe code (say, https://ghc.haskell.org/trac/ghc/ticket/9858) can mean a complete redesign of the patch, if the one in HEAD relies on other changes that can't be sensibly backported.
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