i havent read any of this thread but i am going to answer your question for you.
because automatic transmissions have become more efficient, more reliable, and more powerful than manuals.
manual transmissions while "fun", are a relic. modern semi tractors dont even use them anymore and by a WIDE margin, the trucking industry has embraced it as the superior option.
i know this isnt the answer you want, but its the answer. i know this is a forum for loving old things, and man do i understand that, but, the facts are the facts.