There are other considerations here.

True - 6 axle units are more expensive to buy and maintain.

WP had easy 1% mainline grades (except for the Inside Gateway and branches). As they chose to keep trains moving at 20mph+, there is little benefit to having the extra weight and motors of a 6 axle. SP was another story, expecting trains to pull everything in sight on heavy grades at 10 mph where 6 axles where a big benefit.

A GP40 worked great on the DRGW and WP where they assigned enough hp to keep trains moving at a good pace. But on the SP, a GP40 would derate to 2000hp at 10 mph, making an SD40 that would not derate at 10 mph a better choice. Interestingly, an SD45 will derate to 3000 hp at 10 mph, making it effectively an SD40 at low speeds.

The hp is so high now that the extra axles are needed for both power and dynamics, and to carry bigger fuel tanks and comfort cabs.

WP U30B instead of SD45 might have been more that WP couldn't refuse a bargain. Didn't WP order GP30s but production had already shifted to GP35s for delivery? (GP35s derate at 10 mph also)