I think it is nice because it supports both C and Fortran, and they use the same runtime, so you can do things like pin threads to cores or avoid oversubscription. Stuff like calling a Fortran library that uses OpenMP, from a C code that also uses OpenMP, doesn’t require anything clever.
OpenMP has been around for a long time. People know how to use it, and it has gained many features that are useful for scientific computing.
The consortium behind OpenMP consists mostly of hardware companies and organizations doing scientific computing. Software companies are largely missing. That may contribute to the popularity of OpenMP, as the interests of scientific computing and software development are often different.
I use it sometimes with C++ because it is super easy to make "embarrassingly parallel" code actually run in parallel. And by using nothing but #pragma statements it will still compile single threaded if you don't have OMP as the pragmas will be ignored.
I think it is nice because it supports both C and Fortran, and they use the same runtime, so you can do things like pin threads to cores or avoid oversubscription. Stuff like calling a Fortran library that uses OpenMP, from a C code that also uses OpenMP, doesn’t require anything clever.