C is very different C. There is C where programmer rushes to call fork(), just because he likes the idea of entering room once and exiting room twice. And this program has got excuse for his behavior. He has fork() in POSIX standard. So programming is using fork() here and there even if not strictly needed. Using fork()/exec() instead of more portable posix_spawn(). Using fork()/accept() instead of multithreading.

Then it becomes impossible to introduce database connection pool. Well, probably possible, but in single-process multithread server this is BSc grade job, and in multi-process server this is PhD grade job. libmysqlserver and others maintain some context near to the socket, in opaque way. Hard to pass context between processes. At least, hard to return used connection back to pool in main process. Proxy is also not an easy walk.

On client side although Windows NT had POSIX layer, and XP/2003 still had Interix SFU. But Windows Vista broke SFU. But then Vista has got SUA. But SUA was not binary compatible, required recompilation. And as of Windows 10 there was surely no SUA. Was it dropped in Windows 8? Windows 10 has got WSL eventually, but there was a gap between SUA and WSL. And deploying Windows application with SFU, SUA or WSL part is not easy walk. Writing installer is MSc grade job.

Then comes iPhone OS and Android. They are POSIX OSes. Kind of. But spawning external processes is prohibited or not desirable. This is what was blocking LaTeX adoption on mobiles.