OOTS is a long-running narrative that won't be appreciated fully without starting at the beginning and following the story. Fans of the comic look forward to each new episode to see how the plot and characters will develop, and they love to talk about it with other fans.
That's much of what goes on on sites like ENWorld - RPG fans sharing their interests. It's often hard, if not impossible, to have these kinds of discussions with coworkers, family, or friends who don't share the hobby. Because people on ENWorld read and talk about OOTS pretty regularly, other people who read the forums become exposed to the comic and read it themselves.
Web comics spread best through word of mouth (or the electronic equivalent thereof). All the discussion here about OOTS and the enthusiasm shown for it by its fans is creating a nice audience of ENWorld members. This is where I heard about it for one, and now I and my older son read the comic regularly.
Perhaps folks on other D&D sites aren't aware of OOTS, or perhaps they don't talk about stuff not directly related to gaming, such as web comics. So why does ENWorld talk about OOTS and other sites don't? Probably because this site has a critical mass of readers that the other sites may lack.