They see Lebron as a threat to their favorite player growing up, which for most people is Michael Jordan or Kobe Bryant. Older fans may see him as a threat to Kareem or Magic or Bird as well. Lebron’s greatness is undeniable, his stats and awards rank him among the top 5 in NBA history for sure.
But since its the social media age, people got to pick apart everything he does. If he misses 2 free throws he’s suddenly the ‘worst clutch free throw shooter ever’. If Lebron suffers a cramp and has to be carried off court he is ‘weak and feminine’. If Lebron flops on a foul, he is called ‘LeBitch or Queen James’. If someone else took the last shot in a game (Ray Allen or Kyrie Irving) then they ‘saved’ Lebron’s legacy. People also make fun of Lebron’s Finals record, even though basketball is a team game, and making the Finals is better than losing before the Finals or not making the playoffs, but for these haters it doesn’t matter, they just need a reason to hate Lebron.
With older players not only did they not have social media to deal with, their highlights constantly get played on Youtube etc so when modern fans look at older players all they see is their best moments, so its no wonder that there’s a trend that the more modern a player is, the more likely he is to be hated, and the greater he is (I.e Lebron, Durant etc) the more hated he will be.