Swept!

[VouvrayHead]

"I think I used to get a higher free throw percentage than some of the pros. Some just don't practice it any more--going for dunks and jumpshots and the like"

Many of them that struggle, Duncan and Shaq for instance, have hired specialists and worked for thousands of hours on this skill, yet still struggle mightily.

I think it's the size of their hands. They can't get back-spin. It'd be like a normal-sized dude trying to shoot a big softball--you'd have to balance it on the fingertips to get spin, but that would reduce stability.
Then there's the line-drive factor. The basket is a bit too low to get a good rainbow.

There are big guys that can do it; it's just more difficult.

What I don't understand is why ALL smaller guys don't shoot 85+

By the way, given the options: GO CELTICS!