Essentially, he hasn't been the same player since knee surgery (the summer after sixth man of year).
2013: 17.6 PER, 52 TS%, 27 USG%, 15 AST%, 16 DRB%, 35% 3PA Rate, 25% FT Rate
Since: 13.8 PER, 53 TS%, 21 USG%, 15 AST%, 12 DRB%,
53% 3PA Rate, 12% FT Rate
He is really just shooting threes now (67% of his shots w/ CLE were threes)
0-3 Feet
2015: 56 FG%
Career: 60 FG%
2013: 64 FG%
2012: 66 FG%
So it's clear he is either
A. Much worse at finishing around the rim
B. Not able to create the opportunities he did in his younger years.
Analyzing his dunk data, he was a big-time dunker his first 4 seasons in the NBA (Age 19-22), settled in as an above average dunker (Age 23-26), and is now just an average dunker given his natural, athletic decline (Age 27-29).
The core issue is that guards RARELY dunk in real life (even Jordan was once or twice a game in his youth - which is NOTHING compared to how many shots he was actually taking) but that is the selling point of the game for a lot of people. People want to dunk with guards, and it is still stupid easy to do so. Lance Stepenson dunked TWICE last season and just think of how many times you banged with him last year if you used him (68 Dunk Rating, 50 Contact Dunk, 50 Standing Dunk, he's actually better than JR here, just can't shoot).