I was shown some OCaml code at the university, so learning OCaml later seemed easier than Standard ML. Also I use Debian, and after a quick look at the repositories it has far more libraries for OCaml (-ocaml-dev) than for Standard ML, which tipped the balance in favour of OCaml as the language that I wanted to learn.

Why did I stick with OCaml after that? Mostly for two reasons: there are some libraries that I like (OCamlnet, Lwt, just to mention a few), and the community appears to be more active in recent years (lots of work on build systems, packaging, new libraries coming out, user meetings, etc.).

TBH I never followed the Standard ML community, so I don't know if its similar.