Does postscript require a license? I'd imagine that would be a major hurdle for a free program if it does.
Considering Linux has support for Postscript, I don't think this particular element is as much of a problem versus others. Say for instance support for very specific printers and their capabilities and having that wrapped in a nice GUI for end users.
Bare in mind, free and open source are not always the same thing. There are free programs that are commercial/closed source.
Even if Postscript had a more hardened license etc, keep in mind there are still ways around that. FFMPEG (for audio/video work, Blender, VLC, Handbrake use it under the hood) and one of my favorite audio tools, has codecs for the more protected formats (well at one time that would have included MP3, but that has since lapsed ) and it was still able to exist. Now it was licensed differently depending on what codecs were used in a specific version (same with GStreamer), but it was always open sourced.