I put no weight on people who haven't seen the show saying "he's definitely getting the costume," so I'm not sure what that has to do with anything. I know me (and probably others) said not to rush to judgment about whether it'll appear because they could be saving it, but that's different from saying they're definitely saving it for the end.
As for why it didn't happen, it's precisely because of its importance. They don't think he can slap on the costume because he wants to wear a costume. They have to build it up for some reason as a climactic moment. The show had another focus. Defenders will have to focus on everyone. I'm certainly not saying I agree with them. I'm trying to explain their logic. The fact is, Daredevil spent much of the season exploring the issue of a costume, both from a practical body armor sense and in the symbolic sense. He also wore a disguise throughout the show before he changed the look of his disguise. Iron Fist did none of that.
What assurance do we have that it will happen in season two? Absolutely none. In fact, the only thing we have to go by is Finn Jones not thinking it'll be there in season two (that being said, I don't think he has any more weight than people who said he'll get the costume since the scripts haven't been written). I'll be disappointed if it doesn't happen in season two, but we don't have any assurances. However, there's greater reason to think it'll happen in a season two that can look at this season and think about the feedback it generated than a Defenders show that has already wrapped production. Season two sets itself up well to add the costume both with Danny thinking more about K'un Lun and what it means to be an Iron Fist and because of his interactions with Daredevil wearing a costume.
I've said before that I thought they could easily have fit the costume into episode six and episode 13 without changing much of anything in the story. However, the Easter Egg WWII footage would have lost much of its impact (I realize you probably think it was a waste anyway, I thought it looked cool and fit the story they told very well).