There were a couple good reasons for POB to declare for the NBA draft in 2006, and neither of them had anything to do with his development as an NBA center.
The year POB left Bradley was the first year the NBA instituted the rule that high schoolers had to do 1 year of college (or wait 1 year after high school) to be eligible for the NBA draft. That made the 2006 NBA draft very thin with big men, since the players that were great out of high school could not declare for the draft and had to wait a year, and any good big men who were 1 year post high school already had declared the year before.
If POB had waited 1 more year, and every year after, there would have been a bigger pool of good big men entering the draft and POB probably would have been pushed down and out of the lottery. Plus, after the 2006 Sweet 16 run, and POB's incredible performance against Pitt, his draft value was never going to get any higher.