Actually I have plans to release all of my 15mm tanks in both 6mm and 28mm.
Problem is, no one buys them.
The only 28mm vehicle I make that sells semi-regularly is he Caiman. Polecat sells occasionally, Sabre very rarely. I released the 28mm version of the Red Banner about three weeks ago and didn't sell one of them. So cranking them out in 28mm, which is hugely expensive and nets little or no return, is not a high priority. :-D