Why does Captain America's costume change in all the movies?

Question

As you see in this image, his outfit is different in every film he's been featured in so far.

Why?

Answer 1

In-Universe

We're looking at upgrades. In each movie, we see Cap get "better" gear. His first outfit is barely a costume. Then some colored fatigues. Then he's frozen in ice and gets a modern-day rework made by SHIELD with input from a very excited Coulson:

PHIL COULSON:
Oh, you are. Absolutely. Uh... we've some modifications to the uniform. I had a little design input.

STEVE:
The uniform? Aren't the stars and stripes a little... old fashioned?

Then he spends more time with SHIELD and Tony Stark, both with essentially limitless fiscal and technological resources, that can update his gear for better production and better publicity.

Remember War Machine was temporarily changed to the Iron Patriot design, and even the name was picked because it did better in focus groups.

JOAN RIVERS (ON TV): Same suit, but painted red, white, and blue. Look at That. And they also renamed him Iron Patriot. You know, just in case the paint was too subtle.

Rhodes and Tony Stark are at a bar and they see Joan Rivers making fun of the Iron Patriot suit on TV

RHODES: It tested well with focus groups, alright?

Someone inside of SHIELD could have also been deciding that "this hue of blue is seen as less hostile". Yes, this is pure speculation, but the point is that we've seen numerous in-universe reasons for costumes to change between the various Avengers characters.

Out-of-Universe

Merchandising. Hands-down.

Disney's acquisition of Marvel was largely about capturing the boy demographic of the toy and merchandising market:

The brooding Marvel characters tend to be more popular with boys — an area where Disney could use help. While the likes of “Hannah Montana” and the blockbuster Princesses merchandising line have solidified Disney’s hold on little girls, franchises for boys have been harder to come by.

And merchandise sells, as Disney well-knows. By 2008, the original 5 Disney-Pixar pictures made roughly 10% of their total revenue from merchandise alone. With The Avenger's, we see that it made $1.5 billion worldwide at the box office, but did a whopping additional $1 billion in merchandise sales. Last year, it was forecasted that Avenger's as a brand would reach merchandise sales of $6 billion.

But you don't sell toys by selling the same toy every movie! A new costume or armor or outfit necessitates a new toy, which gives a new motivation to buy the new one.

You'll notice these types of changes with any major franchise, especially those with merchandising tie-ins. The Dark Knight trilogy had a new Batsuit or Bat-vehicle every movie (Rises even led to the camouflage Tumblers). The Transformers films always seem to upgrade Bumblebee to a new prototype model car. The Iron Man films number his armors and are much more obvious about the design changes.

There are probably plenty of in-universe reasons for costume changes that could come up in any movie, but my money is literally on the merchandising explanation.

Answer 2

Yes, merchandising. Biggest reason these days.

Also, costume designers like to be employed, and everyone contributing to a movie likes to make their mark on it. Even without merchandising, costumes change every movie, with few exceptions.