I’ve sometimes wondered why the 300 good report threshold must be met in order to gain access to the last 30 day report detail window. At some point I guessed it was a perk that had the threshold as an incentive, but it’s not really common knowledge, so the incentive theory doesn’t quite add-up.
This is speculation: Maybe the admins don't want someone who has not shown they know how to effectively report posts trying to question unhandled and bad reports.
Quite a lot of my reports have been threads where one user has posted multiple times (usually five or less minutes apart) - at a guess, the admin/mods have looked at the sheer numbers reported and decided not to bother.
More speculation: If you are reporting posts that are old when you report them, your report may not be high on the priority list.