#2 His popularity
Isn’t it always the most vain things imaginable that makes a person hate another? Of course its the same thing that made some fans hate Dale Earnhardt Jr, especially since he was heavily focused on as the sports future for years, but it is also a trope among fans with popular drivers. In fact, its kind of the way all sports have gotten lately.
Think about it! There are a lot of fans that love Lebron James and think he can do no wrong. While those people might be in the majority, there is still a very vocal minority of people that think he is vastly overrated. Same thing for Football and Tom Brady! Some believe that Brady is The Greatest of all time, but others see him as a cheater that can’t win on his own.
For NASCAR back in the day, it was Dale Earnhardt that earned that moniker of a driver that was mega fans with fans, but had a vocal minority of detractors as well. In the end, the trope has existed in almost every single mainstream sport and is even the reason why some fans didn’t really care for Dale Jr during his run in The Cup Series.