Now that you mention it, that is annoying. I was doing split screen with my friend and we thought it was pretty funny that Leia was still in Hoth gear on Tatooine but when you put it like that it sounds a lot worse. A bit of a missed opportunity, although character models are generally one of the most time consuming things to do. According to Rocksteady, a full suit for Batman would take them about a month (I assume they would have one or two people working on it).
I simply don't buy "changing their clothes is too resource intensive" even if they did have the detail and FX of batman (they don't).
The answer is that there is no good answer. It's simply baffling. I am hesitant to ever call any game dev "lazy" because I know they aren't, but in the absence of any rational explanation, I'm just going to call it bone headed.