Looks like it was mostly proximity. Kansas was the southern most of the pair and a large number of pro-Slavery adjutators came in from neighboring Missouri.

Nebraska is further North, when abolitionists were coming force in Kansas they effectively closed out Nebraska to competition (also it was much larger and less organized)

On top of proximity and notoriety, with growing seasons a bit longer and lowest temperatures 10°F warmer, Kansas farms were/are a bigger prize.

It was mostly proximity to the south, but also like 5 people lived in Nebraska at the time.

https://history.nebraska.gov/nebraska-statehood-launched-in-troubled-times/ Simple, Nebraska was one of the union states that didn’t want blacks to vote and thought they were protecting white labor by keeping blacks free but powerless.