It simply isn't a noteworthy enough car for premium status. I'd much rather prefer the Alto Works to be premiumized, but only as an upgrade from GT6 to GT7, not as DLC, due to firstly being an unwise business idea (not much people care for slower, modern kei cars in racing games I've found), and secondly, much more interesting cars really deserve it as DLC more urgently.
I just don't find bland, un-noteworthy kei cars interesting.