Democratic Party of Wisconsin Chairman Mike Tate said it was going to be this way and he meant it. Tate said he believed the party would pick a candidate and avoid a primary. Julie Lassa is apparently the annointed one. Now, there are logistical reasons why each of the other potential candidates chose to bow out. But Tate saying what he said publicly makes it pretty clear that there was an agreement that Lassa would be the candidate.
My personal opinon is I don't see her as being a particularly strong candidate. Secondly, I think this is a tactical error; a hotly contested democratic primary would generate a lot of attention, while Sean Duffy will likely walk away with the Republican nomination. I predicted this is a threefold error; Lassa's a weak pick, democrats deprive themselves of the attention of a primary, and risk voter backlash for annointing a candidate. We'll see.