ReasonML was written in SML then they switched to Ocaml because everyone at Facebook already knew it and caml4p is a thing.

Because it is "just Ocaml", all the weird parts of Ocaml bleed through the reason language design. No operator overloading, nominal record typing, 3 competing standard libraries (plus the all-too-small actual standard library). The list of oddities is rather long.

SML was and is a better choice. SML has structurally typed objects and first-class constructors. No named arguments, but structurally typed objects do a better job at this (like JS destructuring, but more efficient). Anonymous records are possible. A ref type rather than mutable record fields. Only one `let`. PolyML has shipped a multi-process implementation for years while Ocaml's is still in an unending beta. SuccessorML (SML 2.0) is adding some syntactic niceties along with things like module typeclasses (modules should prevent the typeclass hell of Haskell). Most importantly, SML is a standard with multiple implementations and isn't completely tied to the success of Jane Street.