Personally, the reason why the game got boring is the physics/handling. There is no sense of speed and all of the cars feel the same. Also, I found accelerating to be more of a chore rather than a pleasure and that when I had to brake, it wouldn't gently slow down (even when I apply the smallest amount of braking), it would brake weirdly and feels unnatural.
This is all while driving in the bumber view. I find the Cockpit unusable because of the head turning, No indicators, tint in the windshield (I find this very funny since because you have to be a complete moron to tint your front windshield in real life and they have it in here), and most importantly-the lack of an adjustable seat. It really takes a lot of fun out. Those pretty much make the cockpit useless thus making the game pretty boring for me.
Then there's the championships which weren't too hard nor too difficult. The fact that the NPCs talking/repetition/lack of excitement made me have to force myself to do the races rather than enjoying it myself. Finally, the first thing I said when I finished the last cup on Hawaii was ,"What do I do now?" After you finish the championships, other than exploring, theres really not much to do especially with all of the online issues.
Sorry if this seemed like a complaint list, but those are what's making me bored with this game.