One particular practical reason:

# type t = Int : int -> t | Float : float -> t [@@deriving bin_io, compare, sexp];;

Characters 22-23:

  type t = Int : int -> t | Float : float -> t [@@deriving bin_io, compare, sexp];;

                        ^

bin_write_sum: GADTs are not supported by bin_prot

I'm not sure whether it is a real limitation, or it can be fixed, but so far there is a difference.