In this case I think it's mostly because exchange fandom (some of which has read and/or is interested in Percy Jackson) is a lot better at commenting than Percy Jackson fandom in general.

(In my own case, I sort of feel like I said what I had to say about Oracles on the Delphi Express the first time I read it, whereas I didn't see the Thalia story until after I posted it. I am a bit of a fandom hypocrite here because I like it when my betas comment on my story after I post it, but when I'm in that position I can't think of anything to say.)