Characters who become widely recognizable usually wear one outfit, and that outfit tends to be simpler than realism would suggest. Both facts follow from what the design has to accomplish.
The silhouette is tested before the detail
Designers evaluate a costume as a black shape, checking whether it remains identifiable when color and texture are removed entirely.
A silhouette that reads at a glance survives small screens, distant shots, animation at low resolution and a logo printed on a cup.
Detail added on top of a strong silhouette enriches it; detail compensating for a weak one rarely works, because the eye resolves shape before ornament.
Constant wear is a comprehension device
A character who changes clothes each scene forces the audience to re-identify them, which costs attention that a story wants spent elsewhere.
Animation adopted the convention first for practical reasons, since redrawing a wardrobe multiplies work across every frame of every episode.
Live action inherited it because the benefit persists. In an ensemble, consistent costuming lets a viewer track who is where without conscious effort.
Merchandise imposes its own limits
A design must reproduce at small scale in molded plastic, on fabric, and in a single-color print. Fine gradients and complex layering survive none of those.
Licensing teams review designs with reproduction in mind before production locks, and elements that will not translate are frequently simplified.
This is why signature outfits favor bold color blocks and a distinctive accessory. Those elements survive every format the character will eventually appear in.
Color is assigned for separation
Within an ensemble, palettes are allocated so that no two prominent characters occupy the same color space, which keeps them distinguishable in a crowded frame.
The assignment also carries meaning that audiences read without instruction, since color associations are learned long before anyone watches a particular story.
Designers exploit and occasionally invert those associations, but the separation requirement comes first, because a frame that confuses two characters has failed at a basic level.
Changes are made sparingly and deliberately
When a signature outfit does change, the change usually marks a permanent shift in the character rather than a passing mood.
Because the original is so strongly established, audiences read any alteration as significant, which makes costume an unusually efficient storytelling tool.
It also makes reversion powerful. Returning to the original design after a departure communicates restoration without a line of dialogue explaining it.