KingD19
Shai-Gen's Enigmatic Wong
With the way they were locked up, there was pretty much a 100% chance they'd have never gotten free.
Zaheer(powerless) was locked up in a one room cell with no furniture on a plateau on top of a mountain a few hundred feet high. And was guarded at all times by Earth, Fire, and Waterbenders.
Ghazan(Earthbender) was on a wooden ship out at sea with no earth around whatsoever.
Ming-Wa(Waterbender) was locked in a cage suspended over a lava pit(that they probably could have dropped if they felt the need). The heat kept her from having anything to bend and kept her mostly too tired to do anything. Also she has no arms.
And the last one, P'li(Combustion Firebender) is kept in a super deep, super cold prison where there was no way she could even begin to firebend. Most of her time was spent trying not to freeze to death.
No one could have predicted Harmonic Convergance or Zaheer getting powers.
Also, it was Aang, Zuko, and Unalaq who imprisoned them. I'm sure they were trying to veer away from the violence that had plagued the world for more than a century.