I know why!
 I was dumb enough to purchase it so let's see...
1) Collision is a joke...cars will drive through you at full speed making you go into park or near it.
2) Collision is also a joke with NPC cars and walls. Sometimes you'll hit stuff and pass through, other times you'll slam into stuff even though you clearly passed it as the collision was drawn much more widely than the visual appearance of what's on screen (like a semi truck being double it's visual width, or passing through guard posts (some) undamaged.
3) There are some turning issues on courses
4) On replay only you show, no others, so you bounce off mid-air when you did hit something in gameplay.
5) Horrid use of color and design, makes track appear invisible on some stages until it's too late not to nail a wall.
6) Piss poor choice in how the championship mode works, forcing to play a set of tracks and pass all or it doesn't save, though in part it does as duels you pay for and if you can't finish all, or bomb it, you lose cash.
7) See #6 then imagine, lack of time, cash gets drained, can't buy shit... game over.
There's 7 good reasons why the game blows and gets ripped on very hard.
I suppose I could add in they cheaped on the multiplayer too by forcing a card per-person when ridge racer did 6 players off 1 card... but that's not a technical flaw, that's just lack of time/greed/laziness your pick.