Nebraska's recruiting territory includes a large swatch of the midwest beyond its borders. South Dakota is really NU's turf as is part of ND. Even better, the Colorado schools are so underwhelming to local talent that Nebraska can regularly cherry pick the best instate talent from a state of 6 million, and Missouri doesn't get all of its talent either, leaving plenty for Nebraska within a day's car ride.
So yeah, Texas has way more people. But Nebraska competes with no other state university and has (if it has the right coaches) the ability to yank away a big part of the border state talent.