Why do the same bans often occur in ranked play at Bronze and Silver ELO?

1100-1400 ELO is a point where players aren't too familiar with the more difficult concepts of the game yet.

If Amumu isn't banned, who is going to kill him? In a coordinated and balanced teamfight, it's highly unlikely anyone but the ranged AD is able to effectively stop him. (Tanks are usually banned because they are harder to kill.) Many players may not know how to position themselves, or to stay alive so these hard to kill champions may be considered overpowered at this ELO.

Who wants to chase Singed through a mile of poison gas? Who wants to be hit into the air by a scary piece of rock? A lack of competency is most likely present at this ELO so therefore certain champions are able to take advantage of this and contribute more to the team.

At professional levels, more strategy definitely occurs during champion selection, but at this specific ELO which most of LoL's ranked players are in, these bans occur often due to the reasons above.

This are very strong picks if you don't have good skillset in team. If you play Solo Q and your team communication sucks you must consider such bans...

Amumu, Malphite, Morgana - AoE ultimate with Crowd Control effects.

Blitzcrank, Malphite, Singed - Great initiation skills that can't be easy stopped.

Shen - Be in every important teamfight like having Teleport

Darius, Katarina - Skills that are refreshed after kill/assist