Practical considerations and cost are the most likely cause. In the US, few cars are diesel to begin with. The public simply does not buy them. In Europe or rest of world, diesels are more popular. Also, manual transmission cars are more popular in EU/World than USA, so it would be the best market for a diesel. A diesel with 8+ gears requires a lot of shifting. Even experienced drivers may become tired of so much shifting. Plus, the more gears, the larger the transmission physically would be. Considering even an automatic transmission, only in the past 4 years have 8+ gear transmissions been created for commuter vehicles. It would also cost more for all the moving parts in a high gear count transmission. If we consider a CVT, it has only been in the last 10 years that they have even become practical for use in cars with modest horsepower or torque. So their adoption is still extremely small.
Now with the advent of electric hybrids, with just as much or more torque than diesel engines, there is no incentive for any company to make a diesel engine paired with a high count gearbox/CVT when better fuel economy and emissions can be realized with electric power.
I'm assuming you're talking about consumer vehicles and not well established tech like big-rigs or commercial ships (ocean cargo vessels).