In Iran the copyrights of foreign works are not well protected. This lead to a lot of copying: western software and books are/were available for little more than the price of the writable CD-ROM or paper+ink you got it on. Translations of works were slightly more expensive, that the translator did have to get paid.
Since Windows (etc.) was free, and the university thought curriculum based largely on US uni-books, Windows is/was everywhere, even in sensitive environments. Window has the worst track record privacy and security (remember NSA_KEY?), and Iran got bitten by this very hard.
I'm not a fan of Windows but Stuxnet didn't happen because of Windows. Iran decided to spin up a nuclear program and Israel and the US had concerns and wanted to stop it. They had the resources to develop something tailored for this unique situation, which included windows, Siemens PLCs (IIRC), Centrifuges etc. and developed the malware based on their target. Even if their target used a different stack, they'd find a way to achieve the same result.
It's all about price. Attacking Linux will be harder, thus more expensive.
You make it sound easy, if that was the case they'd launch one attack every few months or so. This stuff is expensive, and making it 100x harder means 100x less attacks before the budget runs out.
Beyond Windows security issues, if you (an organization mainly) are enough motivated you can attack other OSes as well. There is anwjole industry about exploiting bugs for more secure platforms.
Since Windows (etc.) was free, and the university thought curriculum based largely on US uni-books, Windows is/was everywhere, even in sensitive environments. Window has the worst track record privacy and security (remember NSA_KEY?), and Iran got bitten by this very hard.