No particular reason. There is an accepted GHC proposal to relax this restriction [1], as well as an accompanying GHC issue [2], but it has not yet been implemented.

Ryan S.

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[1] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0170-unrestricted-overloadedlabels.rst

[2] https://gitlab.haskell.org/ghc/ghc/issues/11671