While we try our best to recruit to shore up holes in platform specific reviewers we can't know how active a reviewer is going to be until they're on the job. It just so happens that our most active reviewers are HD console focused. For reference here's our top ten:
1 - Torillian (65) (note: eat it, Machina!)
2 - Machina (63) :(
3 - Signalstar (37)
4 - Spdk1 (36)
5 - SSJ12 (32)
6 - Naznatips (26)
7 - Mr. Nice (23)
8 - Sensei (23)
9 - Elgefe (23)
10 - O-D-C (22)
Since we review on a voluntary basis the only reviews we try to force on people are review copies we get (not a lot from Ninty platforms on this end) and the biggest 1 or 2 games of each month (again, this is mostly HD games).
I agree that not having enough Wii/DS reviews is an issue, but I think we cover the biggest and most important of them so I don't see it as a big issue unless you want our review archive to have the same platform ratios as the sales userbases of those platforms which I find as an unreasonable expectation.
I would prefer if we had the time/money to buy and play through every version of a multiplat, but usually we don't so we can either have our review out in a reasonable time frame or play/review every platform version of a game separately. We chose the first option.