Since this is a licensed movie product they might not have had enough time to start as early as we think they could've.
While not exactly the same I've worked on a number of entertainment/media projects and it can be complicated. There's approvals, more approvals... The people working on the projects (me, lol) get annoyed at the waiting as well. And it sucks when you have to make changes or the other side just sits on it... They take their time, then want to rush you for completion like "when will it be ready?" Well if you didn't take weeks away on your end for your part I'd at least be closer... That's not counting any number of technical hiccups that can happen during the creative process.
But SS probably had to wait for official documents of the costume to be sent to them, they probably didn't get them early enough. Then having their mock ups approved by Marvel takes more time. Then actual execution (sculpting/fabrication) of the piece takes it's time, I'm sure we don't want that part rushed... I really doubt they wanted to miss the movie release or didn't care about making that date. It probably just wasn't possible given several factors most people not in entertainment or media related fields wouldn't really know about. It's not as simple as "we've seen the costume, now let's make a cool statue".
