Because it's not just another generic kart racer and is indeed much more, it actually stands a chance against Mario Kart
+: Much deeper character customisation then LBP.
+: Karts follow the same amount of depth as character customisation.
+: Tracks on disc are fun, highly polished and very well made.
+: Gameplay is surprisingly very deep and introduces new elements compared to what's in Mario Kart. (A boost bar, an item level upgrade system, car knocking like in F-Zero...)
+: Good challenge that is actually fun.
+: Story mode is an actual story and has interesting and funny cutscenes.
+: Story mode also has additional challenges for every race which unlocks things, including grudge matches. (Special boss races)
+: Biff and Gary the hosts are memorable, funny characters.
+: Online is well structured, sharing is easy and accessible and online racing is mostly lagfree.
+: Visual and sound presentation is very high quality.
-: Biff and Gary get boring after a while, since the gags are always in the same line.
-: Adding things to a queue for downloading creations would of been a lot smoother then having to download everything individually...
-: Like LBP, user created tracks rely on the creator to make it worth playing(this is to be expected however.)
-: Some tiny glitches that occasionally pop up in gameplay.
-: No other gameplay modes besides some sort of racing. (iow No battle mode)
-: Loading times are a little longer then most games.
-: If you do not have the incentive to learn or make something great out of the creation tools, then the appeal might be halved to most.
(REVIEW IN PROGRESS, WILL UPDATE/EDIT WHEN COMPLETED)