All the portals have their own anti-abuse filters, and they are managed by different rules and restrictions on what you can and can't upload., so it makes sense that all 3 are different in their overall workings.
The flash portal has the judgement filter, the audio portal has the approval process (which is about the same as the scouting system anyway), and the art portal has its scout system.
As for why, it's probably to help with the amount of art being submitted. The audio portal's approval system only works for a relatively small influx of submissions (when it gets busy it can take up to 2 weeks for an audio mod to approve a submitted track), and graphic artists are usually able to make and submit art much more quickly than audio artists can make audio tracks.
This would be an unsustainable load on the system if it was down only to art mods to do the scouting, so the idea of getting other artists to help with it was probably generated from that (the bigger the art portal gets, the bigger the scouting base gets too, so it balances out).
I suppose the idea couldn't really be applied as easily to flash and audio, since there's no existing option to upload flash or audio onto your userpage (whereas you can upload videos and pictures). It was probably easier from a site programming perspective to do it the way they have done as well.
