Different armour kits could come as their own products, however after market parts aren't really a trend with modern miniatures manufacturers. They'll sell the base vehicle and its up to you to do what you want with it. If there's a demand - and the market's shown there's not historically - a third party could create one.
I just made a set of ERA bricks myself. Just the same part stuck in a mould and duplicated a few dozen times. It does for field modified vehicles.
Most wargamers won't convert their own vehicles however. For them it then falls into the two categories of people who want these specific variants, and those who would make do. The later I believe is the majority of gamers. Its already a fraction who'd buy this kit. And less so the amount who actually know about which variant is applicable to which theatre. You see plenty of people using whatever's available and not caring if its correct or not. Given that, manufacturers may not see the point in selling these after market kits.
It could be argued that a limited number of customers may not matter, given all the upgrade kits available in other scales. But there isn't really the culture in 1/50th to work from. Nor well, in my own experience, do people seem to care much when you convert or make something from scratch. The reaction's much the same as if you were just doing up the base kit.