Because he isn't the only party involved in this. If he didn't want this to happen, he should've done his best to hold on to the IP.
Anyway, I have no quarrel with you supporting a looney, snake worshipping genius (although I do wonder why; are you perhaps a writer yourself?). I merely fail to see how the inevitable fact that a lot of people have only enjoyed Watchmen cinematically, is somehow evidence that the film should've never been made. (Anecdotal evidence ahoy, but: I myself know two people who picked up the graphic novel after seeing the film. Why would anyone wish for that to never have happened?)
I don't think anyone's arguing that the film is as good as the graphic novel. That's literally impossible, considering the facts that Watchmen was tailor-made for that medium, had more room to explore its themes because of its episodic nature, and, of course, wasn't inherently derivative. No, what I and many others are saying, is that, despite the nigh impossibility of an adaptation being good, the film ended up getting an incredible amount of stuff right. Most importantly: most of the novel's themes and characterisation, but also a lot of its looks, and the juxtaposition between gritty nastiness and explorative introspection.
I mean, shit, can you honestly watch the sequence about the birth of Dr. Manhattan without a chill running down your spine? That was better than it had any right to be, and it felt like it had literally lept from the pages.
For every miscast, weasely looking Veidt, there are at least three perfect casting choices (Comedian, Rorschach, Dr. Manhattan), and for every tacky, unnecessary sex scene there are at least a dozen of sequences that are well-made and unforgettable.
This film was supposed to be shit. And it turned out to be a (somewhat tonally inconsistent but) incredibly solid adaptation. How anyone can see that as an overall loss, is beyond me.
Unless, of course, one just has a personal vendetta against Snyder and this is the way to express that.
TL;DR: It's gatekeeping because I fail to see how Moore's wishes are more important than the cultural enrichment of thousands.