Why are Uber Pokemon allowed in Smogon Doubles?

Question

So I've been playing Smogon Doubles for awhile and sometimes you get nasty suprised coming your way in the form of Uber tiered Pokemon and the infamous Drizzle+Swift Swim combo. Doing some research I found only;
Kyurem-W
Arceus (All Forms)
Dialga
Giratina (Both Forms)
Groudon
Kyogre
Ho-Oh
Lugia
Mewtwo
Palkia
Rayquaza
Reshiram
Zekrom
Xearnes
Yvetal
Are banned from Smogon Doubles. So my question is, why are these normally powerful Pokemon (Blaziken, Excadrill, Thundrus-I and Shaymin-S to name a few) aloowed? What is the reasoning behind them being allowed in doubles but banned in singles?

Answer 1

Double Battles work differently than Single Battles.
Think of it this way, where the battlefield looks like this:

               Azumarill                        Shaymin-S
                                    x
               Blaziken                         Vivillon

Assuming Shaymin-S knows a flying move, both Azumarill and Shaymin can shift their focus onto Blaziken, KOing it and making it not become a threat.

Likewise, Blaziken and Vivillion's Bug moves can be targeted at Shaymin-S, KOing it and making it not become a threat.

In Single Battles, Shaymin-S might not be able to KO Blaziken on his own, leading to a Speed Boost and making Blaziken become a sweeping threat.

All and all, double battles can put more focus on the "uber" Pokemon, therefore having an easier time with them.