It's due to the way clan activity checks currently work.
Files uploaded in the main section of the replays board will count towards activity checks in order to help give a more accurate representation of activity for single player/replay orientated clans.
We didn't want this to extend to multiplayer replays as that match will have already counted towards a clans activity when it was actually played, so the options were to get people to post spars in the main board or create a spar dedicated board.
We determined that there just wasn't enough activity to warrant an entirely new board, so went with the first option.
If you wanted the full explanation.