Why is Cresselia more viable in Double?
Cresselia's support movepool and massive bulk make up a large portion of its viability in Doubles. It literally breathes "support"- Dual Screens, Thunder Wave, Helping Hand, Safeguard, and Icy Wind are all ways it can support its partner (all strategies that are not as viable in Singles due to the fact that only one Pokemon can enjoy the support at once, compared to two Pokemon in Doubles) Because of its massive bulk on both ends of the spectrum, it is also an extremely reliable Trick Room setter, and with Moonlight, it can easily heal itself.
Generally, in Doubles, well roundedly bulky Pokemon excel because of their ability to shrug off hits from both ends of the spectrum, in comparison to the relatively weak defensive prowess of leaning Pokemon. This is because while facing two opponents, it's more likely that the opponent will be able to attack from an angle that the wall is weak to. (Good examples of this include Skarmory, which is terrible in Doubles because of its inability to sponge special attacks [among other things], and Blissey / Chansey, which are also terrible in Doubles because of their inability to shrug off physical attacks.)
Entry hazards are another thing. In Singles, Cresselia isn't used as much because it has absolutely no entry hazard control. Compare this to defensive behemoths like Skarmory and Ferrothorn, which can lay both major types of entry hazards, while Skarmory can even remove them. In Doubles, this doesn't matter, because entry hazards are almost never used and are never necessary. Thus, the main advantage of many singles defensive Pokemon are taken away.
Plus it has a pretty good offensive movepool too