It's not a mystery. The US, the UK and France ratified the Budapest memorandum. Ukraine gave up their nuclear weapons in return for security assurances.
No such treaties existed for Iraq nor Afghanistan.
No such treaties existed for Iraq nor Afghanistan.
Sure, there are words saying "the Russian Federation [...] reaffirm their commitment to Ukraine [...] to respect the Independence and Sovereignty and the existing borders of Ukraine" but what if they don't? tough luck. No one promised help, there's no built in penalty mechanism, nothing.
(Also, I uploaded Volkov Commander to SIMTEL 31 years ago and the ignorant asshole running that site reported me to the university for pirating Norton Commander and they banned me from the university VAX leading me straight to Linux. Funny how that worked out.)