Low budget.
Why do cartoons characters were the same clothes and not different ones?
It saves development costs. If Cartoons were to wear different clothing every episode it would require them to make a new version of that character every single episode.
Corey also makes a very valid point as well.
Not a lot of clothing stores. Artists probably forgot to make them as the main focus is hopefully the over-all animation or the story.
Charlie Brown without his striped shirt
Wouldn't be Charlie to me...
Someone else already said to make the character recognized each time you see them.
The characters in Kim Possible, Adventure Time, and I believe As Told by Ginger had characters with different outfits. Especially Kim and Marceline from the first two shows.