The main factors I believe are (in no particular order):
1 - commercialization of the industry with influence of publishers/hype on reviewers
2 - lack of real clarity for reviewing principles in videogame industry vs say film (with films, no matter how much something like Transformers 2 makes, it's still gonna get ripped critically)
3 - heavy focus on genre titles results in higher number of good games that aren't innovative but are decent as they are built on readily available solid tech and use a 'paint by numbers' to put in accepted gameplay mechanics
4 - rising cost of development means less stinkers as those that publish stinkers will lose money and exit the game (this is why Wii sees more shovelware I believe, as well as it's higher install base)
5 - for PS3/360 PSN and Live offer a less reviewed and obvious route for smaller titles, some of which are very good, others of which are average at best
Still, there are always the wacky outlier reviews keeping things interesting - and infuriating those who track metacritic averages as the true sign of a game's worth.
Try to be reasonable... its easier than you think...
