Fair question, Enzo.
Convicts are sent to prisons according to the length of their sentences, severity of their crimes, and the perceived danger they pose to society and to other prisoners--not necessarily their status in The Mob. Gotti was convicted of planning the murder of Castellano, and was therefore sent to a very tough prison. Others were convicted of somewhat lesser crimes, and were therefore sent to less stringent prisons. Before RICO, most mob guys were convicted of relatively minor crimes, and were sent to "easy" prisons like the Federal lockup in Lewisburg, PA, where they ran their own wing of the prison and lived better than the others. An accurate portrayal of life for Mob guys in Lewisburg is found in the movie, "Goodfellas."