Because, pre-Nidoqueen at least, RU stall was the most viable stall archetype out of any of the tiers? It dominated the ladder immediately prior to Nidoqueen's drop. Then Nidoqueen destroyed most forms of stall and in case you hadn't noticed, which you likely didn't due to likely not playing the tier, Nidoqueen was a recent suspect. Cresselia invalidates any form of offensive with such ease, without having to try particularly hard. That on top of having the ability to tweak her moveset to beat certain checks or counters makes her really frigging good.
Essentially, Cresselia doesn't just beat a lot of Pokemon, it beats an entire playstyle, bar a small, select group of Pokemon. That's broken in my books, from a defensive perspective.
Offensive suspects are obviously judged much differently.
EDIT: Don't like quotes like this but it's necessary this time I think. Plus I'm lazy/busy atm
Originally Posted by Fat Lee
stop trying to counter Cresselia with offensive mons, there's your problem right there - very few can wear her down before she beats them and even fewer like getting paralysed. Nasty Plot Slowking dances all over Cresselia,
252 +2 SpAtk Life Orb Slowking (+SpAtk) Surf vs 252 HP/116 +1 SpDef Cresselia (+SpDef) : 36.71% - 43.24%
Hardly walking all over Cress, and if it carries Toxic (admittedly unlikley) Cress can quite possibly stall out King
252 +6 SpAtk Life Orb Slowking (+SpAtk) Surf vs 252 HP/116 +3 SpDef Cresselia (+SpDef) : 44.14% - 52.03% In the worst case scenario
252 +6 SpAtk Life Orb Slowking (+SpAtk) Surf vs 252 HP/116 +4 SpDef Cresselia (+SpDef) : 36.71% - 43.24% A more likely scenario given Cress outspeeds
and a simple Toxic Slowking hard walls any Cress that lacks Sub or a Toxic of its own. Ferroseed gets risk-free Spikes on any version that doesn't use Calm Mind or HP Fire , a decently bulky Pokemon like Munchlax (crap), Mandibuzz (poor) or Steelix using Toxic + phazing move (Steelix can rarely fit Toxic) can deal with her in all situations sans last Poke Sub Cres. A decently sturdy Pokemon using Substitute and without a weakness to her coverage moves can use her as set up bait - Jynx, for example, cannot have her Subs broken by any Cress that lacks a SE Hidden Power and even then it can just use Lovely Kiss.
Not entirely accurate. Gallade for example will lose to Cress even if it carries Substitute, and it has 115 base SpDef!
Murkrow can just Perish trap her for the gg, there is loads of stuff really but you are going to struggle if you're using a bunch of fast, frail set up mons and if you keep giving her free switch-ins because she has been thriving against that sort of team for years now.
Not sure what to say except that Murkow carries little viability against the majority of RU.
and stop thinking that hitting her super-effectively is some sort of holy grail, she isn't THAT inconvenienced by Dark or Ghost types on account of their weak STABs - Escavalier on the other hand...there's a full stop right there unless she's using HP Fire which has sone pretty unimpressive damage output against escavalier depending on her SpA investment (even at max SpA and +1 Escavalier survives the hit 2/3 of the time)
It has been mentioned before that Cresselia beats even her supposed offensive counters, and that she 2HKOs Escavalier with HP Fire, meaning catching it on the switch once or hazards accrual equals a dead Escavalier.
Sorry for that ugly thing but just wanted to address a few things that didn't paint an entirely accurate picture.