There is a market for manual transmission trucks.
Not enough of one to continue production in the half ton class. The big three offered them at zero cost and the take rate was under 10%; in some cases it was under 1%. There was no business case that would justify continuing to produce them, particularly in the era of CAFE (manuals do worse on CAFE). If people wanted manuals they should have bought them in 2000-2010. They didn’t, so they’re gone now.