+10 for bad windows support, i think this is a key and weirdly underestimated reason
just to give an idea how bad, until recently, you could not just go to ocaml.org and download ocaml for windows, you had to either download one for mingw or wsl
so for many it was just not installable, i.e. for many we didnt have ocaml for windows, until very very recently
The Windows support is bad because OCaml is a PL designed by people who are deep in Linux life. Windows is not something that keeps them up at night. (Which isn't to say they didn't try, just, you know, not as much as it takes)
One of the things people often neglect to mention in their love letters to the language (except for Anil Madhavapeddy) is that it actually feels UNIXy. It feels like home.
If that's what you use as your yardstick of what's Unixy, then I guess you don't consider "find" to be Unixy, in spite of being one of the early Programmer's Workbench tools.
Short options were a compromise to deal with the limits of the input hardware at the time. Double dashes were a workaround for the post-dash option car crash traditional Unix tooling allows because teletypes were so slow. There is nothing particularly Unixy about any of these options other than the leading hyphen convention.
Caml was developed in 01985, Linux was first released in 01991, and Caml was extended into OCaml in 01996. I don't think the developers were using Linux at the time; SunOS 4 would be my best guess. I didn't work on it, but I was in the sort of internetty environment that OCaml came from, and I was using Solaris at work and got my first Linux box at home. As another commenter mentioned, the command line follows Unix conventions rather than the Linux conventions from GNU.
just to give an idea how bad, until recently, you could not just go to ocaml.org and download ocaml for windows, you had to either download one for mingw or wsl
so for many it was just not installable, i.e. for many we didnt have ocaml for windows, until very very recently