There are a couple main reasons.
In college I had a professor from Kansas. He said there are 3 parties: Democrats, Moderate Republicans, and Conservative Republicans. The moderate faction will break with them Dems if the conservatives go nuts (see Kelly, Laura). Right now, the moderates are pissed at the conservative faction (see Brownback, Sam, and Kobach, Kris).
The other main reason is that Kansas (at the state level) has a history of Democrats winning governors races and a few in Congress. Besides Ben Nelson and Brad Ashford, Nebraska doesn’t really have that.
Exactly this. The Dems rightly focus on Kansas vs. Nebraska because of political history. While similar demographically, the political dynamics of both states could not be more different.
The Republican party in Kansas is one of the most fractured state parties
in the nation and has been that way for decades. The moderate sliver, when fully in line with the Dems, can deliver a Dem a narrow victory. The moderate faction also increasingly looks like the suburban Dem voter profile that's hardening across the nation after 2016. Romney-Clinton voters.
Just look at the state party's
wiki page. It's an ongoing decades-long feud between the business friendly moderates and the Bible-thumpers, to be reductive.
