Maturity, in general.
And then it had a lead-on:
Bad BBIQ and lack of fundamentals
Uncoachable
Lack of motor
Unwillingness to play defense.
Nothing really out of the ordinary, just a mixture of being in the wrong place at the wrong time (in the basketball sense), and bad habits. He also suffered from the perception of not having a position either, which was largely due to his weight where he was like 215-220lbs at 6'11", coming out of college, and didn't play with any real physicality. I don't think teams really knew what to do with him.