Efferding syndrome. Squatting a bazillion pounds for couple of shallow reps doesn't do a whole lots for leg size or shape. Dude needs to use decent form and do sets of 20 or something.
What makes you think he doesn't? Some people just have bodyparts that are mediocre, compared to other ones. With Jackson, it's his legs. They're good, in and of himself. But, he has more upper body mass.
Considering he's won the Nationals, three pro shows, and is a top-10 Olympia placer, I'd say Johnnie knows a little something about training his legs.