My guess as to why:
Two reasons.
First, I doubt if they sold enough manuals to warrant the costs involved in developing and certifying the emissions to make it profitable.
Second, I expect emissions certification is a lot more difficult with the manual. With the automatic they can simply program out conditions that result in excessive emissions.
Remember, too, the manual D-max is derated compared to the automatic, so there must also be some durabuility issues.
The Constitution needs to be re-read, not re-written!
If you can't handle Dr. Seuss, how will you handle real life?
Current oil burners: MB GLK250 BlueTEC, John Deere X758
New ride: MB GLS450 - most stately
Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510