In doubles both players have to work together to combat other players. Lots of things are different because there is little to no "Neutral" game. You can't read a shield and go for a grab because their teammate, if they are smart, will throw out a hitbox. It's the fact you can always throw out both Paper and Scissors to win it that destroys the Neutral. Doubles is more combo based as someone can pick up when you go into endlag.
Not to mention, combinations of characters will make a huge difference, since you might find a combination that works really well due to syncing.
In doubles, there's a lot more to worry about and you need a lot better reactions. In singles, there's less to worry about because it's all right in front of you, but, you need to have a better mindgame and punish game. This is how I understand it.