I'd say, it's not so much the API itself, as much is there is just ONE API - POSIX. Files, processes and signals. This it the root of a very simple design philosophy, that much *NIX software adheres to.

Most thirdparty libraries provide a C-library that can be linked to an application - but it also provides a small tool that implements all functionality. Input values aren't "resources" or instances of this or that object. They are one of: Strings on the commandline, a piped stream or a file. And most often, all are possible. Output is a integer return value (typically just true or false), another stream or another file.

Windows does not have that single, simple API that everything adheres to, so there's no one culture of Windows development. Many of the APIs that exist in the Windows ecosystem, e.g. WMI, that I've been using to script IIS control, is perfectly well equipped to become a "POSIX-killer" - if it would just become pervasive. WMI is next to worthless when I can use it to control IIS, but not some other feature, and I can straight away forget about talking to the Win32 API altogether from Windows Script Host, and even in C#.net it's unnecessarily difficult.