Gametrailers is all about serving audio and video about games to people. It makes sense for it to have a bias toward games that have shiny videos or big marketing budgets (to release lots of trailers).
Many Wii games are very tactile. You can't appreciate how good they are through a video camera. You have to feel the satisfaction for yourself. For this reason, even if the Wii had HD-quality graphics, Wii trailers would still suck.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.