AFAIK the reason is solely error messages: the customization available with handwritten parsers is just way better for the user.
https://github.com/NixOS/nix/blob/master/src/libexpr/parser....
https://github.com/NixOS/nix/blob/master/src/libexpr/lexer.l
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AFAIK the reason is solely error messages: the customization available with handwritten parsers is just way better for the user.