Why, just because Alan Moore is an elitist prig? Who cares WHAT he thinks--it's DC's property and his opinion no longer matters. I get where he's coming from, but he doesn't give a tenth of a mess about any relationship any of his fans have created with these characters at this point. Plenty of others do.
I respect Moore's ability, but not his arrogance. Yes, you can't translate all of the innovations to the comic medium he made in "Watchmen" to other media, and yes, any attempt to do so will be inferior to "Watchmen" because the odds that a Snyder or anyone else would similarly try to innovate the translated medium anywhere close to the extent to which he did is nil to zero. Who cares? Mostly Alan Moore and not many others. Do we really miss and return to re-experience his innovations to panel-based art that much? No--the vast majority of us re-experience Watchmen because of the story and the characters. You can still tell good stories with these characters, and while it won't be Alan Moore's Watchmen, it's still possible to do them well. He's not the only writer capable of penning an anti-superhero story.
Nothing to do with Alan Moore. Watchmen is a complete work with no loose ends. You can't spin off the characters because they aren't 'superheroes' that can go off on further adventures. Their arcs are complete.
You wouldn't spin off Hamlet would you? (Oh wait... I did like Rosencrantz and Guildenstern are Dead...)