Smaller platforms (which Linux and Mac OS are, compared to Windows) draw less interest (both in exploits and scrutiny against exploits), and fragmented platforms (Linux) are harder to code for (SystemV? systemd? something else entirely?). I guess that it would have required more effort - but at this level of involvement, I would say that the result would have been achieved regardless.
The stuxnet dev team managed to get their hands on the exact same centrifuges as used in Natanz, so getting replica Linux control systems would’ve been a walk in the park.
Also, “less interest” is irrelevant when we are talking about nation states picking a specific target and throwing considerable engineering resource at exploitation.
