Check out the most popular music today. Like the top ten songs currently. Do you think those are really the best songs out there?
Popularity is mostly driven by either trends or momentum.
Yes! To add to that, the question itself is wrong. We should be asking, how is OCaml able to be so good without being popular? People get the whole thing backward.
The popular languages are typically popular first, then get good later as a result of that popularity. They have to work, they have to be good, they're too big to fail.
This is what happened with Java. It was marketed like crazy at first and only later got refined in terms of tooling and the JVM itself. The R programming language was a mess for data wrangling, but once it was popular, people built things like the tidyverse or the data.table library. The Python ecosystem was disaster with all different testing packages, build tools, and ways to create and manage virtual environments, until the relatively recent arrival of uv, more than three decades after the creation of Python itself. And then there's javascript that's had more money, blood, sweat, and tears poured into it to be improved in one way or another because that's what practically anything running in a browser is using.
Many of the most popular languages are also the most hated. Many of the more niche languages are viewed the most favorably.
It is easy to dislike something yo are familiar with, and easy to be overoptimistic about something you don't know as well.
"the grass is always greener ... "
The reason for why OCaml is not more popular, thus, is that this subset is small. The reason for this may be either (a) habit or (b) it's not that much better than other languages. I'm gravitating to (b). OCaml guys seem to be quite dogmatic for the wrong reasons.
Also it didn’t employ a marketing team to work on outreached and write fancy comments here and some people who have used it for 10 minutes are apparently offended by the Pascal-like syntax and can’t stop discussing it on every Ocaml discussion making every actual users tired.