I think it's because of the length, but I don't know what the limit is.
Edit: Strangely I can't quote/reply to my own post either. However, I could reply to this before adding this edit, so now I'm curious if I can reply after adding more text.
Edit 2: I'm now poking at it it to see what the rules are because I'm curious.
Edit 3: Yeah, it's based on length. After inserting a lot of dots I couldn't reply to this post.
Edit 4: Actually, that's reminded me,
@Secret Asshole have you watched Ping Pong?
Edit 5: Apparently you can't quote or reply to posts longer than 1008 characters. I don't know why this is. Sorry to the mods who can see the edit history for this post and have a screen filled with full-stops.
Edit 6: I want to be meta. This post is 1008 characters long and should be quotable. It also has 6 edits.