Because the fascist-leaning free-software devs feel threatened by codes of conduct. Suddenly they can't harass women or call someone a n-word or a re-word for making a mistake in a patch, or for daring to ask a question. It was a common issue in many circles, where some communities were known to be more welcoming than others, but i don't believe the "code of conduct" as we know it (beyond usual terms of service or association bylaws) was a thing back then in the free-software communities.

Following the Gamergate anti-feminist/alt-right harassment campaign, most communities have been forced (over the years) to "pick a side". Some like Weboob made sure bigoted "jokes" were welcome because noone was going to police their free speech, so the package was removed from Debian and years later it turned out one of the biggest contributors was an actual nazi (who donated a fortune in Bitcoin to alt-right movements worldwide right before taking his own life) so finally the project is being renamed. Others, like the Tor project, made sure very early on to create an inclusive space for dissenters, and in general for people who do not fit in the very narrow boxes established by our society.

Then, there's the BDFL project, where some (admittedly skilled) people end up governing an entire project as a benevolent dictator. I would argue it's impossible for a single person to be continuously-welcoming, but in the case of software development where bug reports and patch reviews can eat a lot of your mental energy, having a BDFL is usually a red-flag for a toxic community. Both FSF and Linux projects have undergone serious governing crisis around their BDFL (rms and linus respectively). The former has been removed from head of FSF (though remains head of the GNU project to my knowledge) while the latter has taken time off to reflect, and apologized for his behavior. Both of these events were interpreted by the far-right either as an overreach of "social justice warriors" putting heads on spikes for no reason, or as an actual conspiracy theory involving secret services "eliminating" the BDFLs in order to backdoor the software they were maintaining.

So in general blaming "social justice warriors" (eg. left-leaning liberals) for our problems in life is factually wrong, and takes place in a wider fascist context (just like the nazis were blaming the "communists" and the "jews"). But specifically in software development, having a code of conduct is fine (when it's not just an appearance), but not having a formal code of conduct is fine as well if the team is small and critical of their actions/privileges. But some projects explicitly refuse to have some for the wrong reasons, and this citadel project has this one we've been talking about which is like an anti-code-of-conduct.

I could go on for hours about how a code of conduct does not necessarily create a safe space, and how no space is really safe and we need to stay critical of the most subtle forms of harassment and discrimination. But the point is, besides the practical guidelines contained in the Code of Conduct, simply having a CoC is a sort of signaling mechanism for an intent to build a decent community, and that's without a doubt a good thing.

Here is an example from ~fr for a Code of Conduct, although we don't exactly like to call it that. Do we really have to "code people's conduct" (a term borrowed from the world of business) to let them know fascists and masculinists (among others) are not welcome on our server? Anyway i hope i answered your question by providing context, sorry it took a while :)