They really didn't, actually.
They really actually did.
The stuff you keep pushing about political interest in Texas is distraction. Banks had satisfied Seward and Lincoln by taking Brownsville. The cotton pressures existed and speculators took advantage of the campaign, but that was not the justification that was put forth by Sherman, Halleck or Grant.
You seem quite impressed with Johnson's book. So I will quote it to you. Johnson wrote "there is evidence strongly indicating that the administration did not regard expanded operations in Texas as being of pressing importance" but Halleck "wanted to attack by way of the Red River and northwestern Louisiana. His views were the result of his considered
military judgement", according to Johnson. See also what I quoted from Johnson's book in post #23 above. In the end it was a military campaign pushed forward by military commanders for military purposes.
Halleck had declared that "The best military opinions of the generals in the West seem to favor operations on the Red River". He pushed it for the
military reason that it would better secure the defense of Arkansas:
"It was at one time hoped that you [Steele] might co-operate with General Banks in holding that line, and thus secure Arkansas and Missouri from further rebel raids, but when General Banks changed his plan of operations to the Gulf coast an advance on your part would have been extremely perilous, and you acted wisely in occupying the defensive position of the Arkansas.It is hoped that measures may hereafter be concerted between yourself, General Sherman, and General Banks to drive the enemy entirely out of Arkansas and then occupy the line of Red River, which is shorter and probably easier of defense."
Sherman was very eager for the campaign, an attitude that had nothing to do with Texas or pressure from business interests. In the months prior to the campaign he had made the following statements:
"The only real battle needed in the Southwest will be at Shreveport, and for it we want high water in Red River."
“In my judgment, we have two more battles to win before we should even bother our minds with the idea of restoring civil order—viz., one near Meridian, in November, and one near Shreveport, in February and March next, when Red River is navigable by our gunboats.”
"If the admiral will agree, I will myself collect at Memphis and Vicksburg about 8,000 men and go up Red River as high as the water will permit, and make them feel their vulnerability."
"I will take all the force I can collect at Memphis and Vicksburg and go up the Red River as high as Shreveport, and make that rich country pay in gold or cotton for all depredations on our river commerce."
Porter would later testify to Congress that "The Red River expedition was originally proposed by General Sherman and myself" and John Hay (one of Lincoln's secretaries) wrote in his diary that it was all Sherman's idea. Sherman wanted to lead the campaign himself when he found that Banks intended to go along and he didn't want to serve under Banks so he backed out.