First, I want everyone to examine the metacritic aggregates for PSP and DS games. The PSP list can be found here:

http://www.metacritic.com/games/psp/

While the DS list can be found here:

http://www.metacritic.com/games/ds/

You will notice almost immediately, I suspect, that there is a grand total of 5 games (4 for DS, 1 for PSP) that have achieved "Universal Acclaim" with a metacritic average over 90. Comparatively, the Wii already has 8 such games, the PS3 has 10, and the 360 has 18. Any one of these systems has more than the DS and PSP combined. This evidence is stark and inarguable, I think.

So what is the cause? I think there are two reasonable explanations: either the handheld systems simply have a dearth of well made games on them, or game reviews have a noticable bias against handheld games. In your opinion, what is the cause of the disparity in how these systems are reviewed?

I think hand held games are held up to their console counterparts by reviewers.  No body takes specs into account.

Well that and the games that traditionally get good scores don't work as well on handhelds like FPS and the like.