So why don’t they just divert the water further upstream via an aqueduct to the reservoir instead of pumping?
That type of power is called "run of the river" hydro power, and is used extensively in the Northwest and somewhat in the Northeast. This diversion is used to generate power when the river flow is diverted, and is especially effective at times of heavy runoff. However, again, environmentalists delay the construction, fight the construction, regulations strangle the construction, and even the Tellico Dam in Tennessee (the protypical enviro-blocked project that started the ball rolling for obstructionists) was delayed for a long, long time, but ultimately built, once the Snail Darter was determined to be less important than the people (immediately when a judge ruled in favor of the project, it was started and Appeals did no good).
Pumped storage is used where flow is less regular and strong, and the water is available, but not enough current is available to run the hydro-turbines. The pumped storage reservoirs are on mountain tops, to provide enough "head" pressure to run turbines at the base of the mountain (which then discharge that water into the rivers below), and the water from the river is pumped up the mountain to re-fill the reservoir.
Too Roman.
The company is based in Nebraska, there is not much grade there, you may be talking about diverting water from another state to get adequate elevation.