Iirc Delphi didn’t have threads, sockets, or OS integration (signals, file watching …). So it wasn’t suited to systems programming ie servers and services. It nailed gui applications, and that was a lot. Maybe freepascal has threads and sockets but imo it was too late.
I have written a number of services in Delphi, some 20 years ago, all works fine.
Eh, sounds like that wouldn't be a problem for education purposes as the parent suggests? You need to be doing some really specific to leverage threads/file watching. And people probably use C to teach threads anyway.
Of course, being a good teaching language probably doesn't make the language popular or even survive. Python is so widely used not necessarily because it's simple to learn but because of its ecosystem.
Delphi 2, the first 32bit version of Delphi, had all of this. Some, like threads, even had wrappers (TThread), but Delphi came with Win32 bindings out of the box so all Win32 functions were available too - and it came bundled with documentation for the APIs. In addition, calling out to a DLL was trivial so even if a function wasn't available, you could just define it. Pretty much anything you could do with a C compiler was possible with Delphi 2 too.
Free Pascal obviously has all of that stuff too.