I'm guessing it's based on the idea that has been getting a lot of pub this year that Lebron can guard all 5 positions on the court given his body. That may be true. However, he doesn't guard all 5 positions particularly well. He can guard some point guards but not others. He can guard some big men but not others. And he is totally reliant on athleticism, the only time I really see him using timing and skill is on chasedown blocks. I just don't see him display his basketball IQ on that end. (For example, there's nobody else I'd rather have chasing a guy going for a breakaway dunk or layup; but if I had 1 guy back in a 2-on-1 break, I'd actually rather have Bird than Lebron back there.)
As far as I'm concerned, when it comes to guarding multiple positions, Pippen was better on ball or shutting down any wing scorer. I'd take him over Lebron just as the small forward on my all time All-Defensive team. And I'd say Artest in his prime was better than Lebron is right now at defending 3s and 4s because he knew (and still knows) how to use his strength and disrupt opponents way better than Lebron does. Same with Bowen. guava wrench is right that Lebron is making strides there, but it's just with brute strength. Artest used that strength but also did it smarter. And Bowen didn't have nearly the strength but could be effective against bigger players based solely on intelligence, leverage and timing.
Off the top of my head, I don't think Lebron even makes my Third Team All-Defense All Time:
First: C Bill Russell, PF Tim Duncan, SF Scottie Pippen, SG Sidney Moncrief, PG Gary Payton
Second: C Hakeem Olajuwon, PF Dennis Rodman, SF Ron Artest, SG Michael Jordan, PG Walt Frazier
Third: C Ben Wallace, PF Kevin Garnett, SF Bruce Bowen, SG Alvin Robertson, PG Dennis Johnson
Tough to leave out guys like KC Jones (made the Hall when it seems his major contribution to the Celtics dynasty was on ball defense but I never really saw him so can't justify putting him over guys I've at least seen highlights of), Cooper, McHale, not to mention all the elite centers like Robinson, Mutumbo, Ewing and Mourning.
Lebron might be better all time than maybe Alvin Robertson or DJ, but he's behind all those big men. Truthfully, I'd guess that nearly all the top 10 defensive players of all time are interior defenders - a strong big man can really just impact the game much more on that end than any perimeter defender can. In fact for my top 10, I'd take the 6 centers/power forwards on that list, Pippen, Robinson, Artest, and maybe Moncrief. And now just saying that, I have a hard time convincing even myself that Moncrief had more defensive impact than Ewing or Mutumbo. Lebron ranks behind Duncan, KG, Wallace, Howard and Artest just among active players.