I understand that Marvel Television in its ‘street-level’ heroes tends to seek a more grounded, realistic esthetic than those typically employed by Marvel Studios.
That probably has a lot to do with why of all the Defenders only one, Daredevil, has a costume (which is more in the vein of tactical armor than a costume, per se).
Jessica Jones and Luke Cage wear civilian clothes, as does Iron Fist (at least in the first season of his series).
And for awhile I thought that the latter in his civvies that might be a good decision, till I saw this image from the series.
That’s Johnny Yang as an ‘Iron Fist’–which is less an individual than an honorific, though only one seems to exist in any given period of time–and he looks pretty awesome.
The way they muted the colors and gave the costume a very real-world feel works really well, and I would have been glad to see it in more detail. It was technically in the series, though the footage of it was so (deliberately) blurry you couldn’t make heads or tails of it.
Maybe they kept it under wraps because Danny Rand (Finn Jones) in–hopefully The Defenders–tries to capture some of his lost history, and dons the costume as a result.