We had enough quantity for them to do a special order. I think they realized that they couldn't just shave 20mm off the mounting pad of the 16x8 ET0 to make it work. They would have to take the 16x7 +0 and modify both the barrel width and the mounting pad to get an 8" +20 to clear Miata brakes. It probably just wasn't something they're interested in.
Plus the 16" Grid V that is forthcoming should come in ET20 - so maybe they wanted to encourage sales of that wheel which is already planned for production.