What is the rationale regarding no manufacturers selling half ton bakkies any longer?
It can't be lack of sales because they are hugely popular in South Africa
The bold bit is one of the big problems. Half-tonners were niche to only a handful of markets.
1 - The rise of the shared modular platform killed region-specific niche vehicles, and the half-tonner was one of these region-specific vehicles. You have multiple manufacturers sharing a common platform across multiple segments. Right now the most popular segments are crossovers and hatchbacks and manufacturers can build these vehicles using a common platform. A great example here is Toyota and Suzuki sharing a common platform across the Baleno/Starlet (hatchback) and Fronx/Starlet Cross (crossover). Half-tonners being niche vehicles that were popular in small(er) emerging economies (South Africa, Brazil, North Africa, parts of the Med and Eastern Europe) do not fit in with this global, modular approach to modern car-making where car makers try and minimise and share costs in developing vehicles.
2 - The rise in popularity of the crossover created a substitute for the half-tonner; the panel van. Nissan, Renault and Chery offer panel van variants of their popular Magnite, Kiger and Tiggo 4 crossovers. You already have the popular crossover, so instead of making a purpose-built half-tonner, just make a panel van out of the crossover. It requires even less retooling and work than turning a hatchback into a bakkie as you really just pull the back seats out, put a net behind the front seats and maybe stick a rubber mat/board on the floor of the now expanded loading area.
3 - Modern vehicles are growing bigger due to safety and emissions standards and comforts demanded by consumers. These shared platforms have outgrown the whole idea behind a half-tonner, which was often just a cropped and scaled back hatchback that didn't care for safety. Hence manufacturers opt for panel vans based on their crossovers. The platform is there and it ticks the safety, emissions and comfort boxes.
4 - The rise in popularity of the Japanese kei vehicles, like your your Super Carry's and Eecos, have filled in the bottom half of the market once occupied by the half-tonner. While there is hypcorisy here, kei vehicles are hardly safe, the kei market in Asia is enormously popular and this popularity has spread to the rest of the emerging world, Africa included, so even less reason for manufacturers to develop a niche half-tonner when they can just import cheap-to-produce kei cars that sell like hot cakes in larger markets elsewhere in the world.
TL;DR - The rise of the global car platform killed the niche region-specific vehicle, and half-tonners were a niche region-specific vehicle. It is cheaper and easier (and safer) to turn an entry-level crossover into a panel van than it is to carve a hatchback into a half-tonner or build a purpose-built one. Hugely popular and cheap to produce Asian kei vehicles fill in the bottom end of the market once occupied by the half-tonner. It is all about the $$$.