It's still early and the game hasn't been revealed for that long. A lot seems to be unfinished/work in progress. I can imagine them working full throttle on the still unrevealed characters, their models, their default costume and fighting mechanics. Alternate costumes are probably not the priority right now.
Let's optimistically assume there are 30 charaters in the game and let's pessimistically assume they get just 3 costumes each, then that's already almost 100 costumes that they have to create.
Now that costumes will also be of higher asset quality and detail, they also take a bit more time to create. Even if they reuse old costume designs (which I have nothing against) and partly reuse old assets (which I have a lot against) that's a lot of work.
I think we should keep our expectations low in regards to the number of alternate/unlockable costumes. We are spoiled because of Ultimate and Last Round, vanilla DOA's never were that bountiful in regards to costumes.
Alternate costume reveals are imo also more interesting closer to release of the game. They will spice up the last couple of weeks waiting and late trailers, when we already know most characters. Spotting a new costume in there will be pretty cool.