That may be because he’s the Justice IG and not the Intelligence IG. I’m actually expecting an Intelligence IG report at some point in the future as well.
Horowitz’ first report was narrowly focused on Comey leaking classified memos. Very narrow.
Next OIG report will be much broarder but still can only be about people who are employed by DoJ/FBI or were at the time, not sure of exact details. But Horowitz couldn’t investigate Clapper, Brennan etc. He could certainly pass on info he obtained to the right person in another agency and most likely did if he ran into anything.