Short answer: They keep moving there and too many of them vote Democratic
The documentation?
Well, start with deep-blue Madison's successes.
No: it's to grab power away from local governments and elected Democrats and restrict voting there because too many of those ideal, productive residents voted for Democrats last month.

The documentation?
Well, start with deep-blue Madison's successes.
Hiring, wages and profits all rise, Madison-area business leaders say in surveyThen take a peak at who wants to live there:
Why Madison, Wisconsin Attracts More Millennials Than Any Other CityAnd, finally, how they just voted:
Report: Tony Evers carried young Wisconsin voters by 23-point marginIs the GOP's answer better policies?
No: it's to grab power away from local governments and elected Democrats and restrict voting there because too many of those ideal, productive residents voted for Democrats last month.
And when offering up one of the GOP's talking point justifications for the [power grab] bill to The New York Times last week, Vos came up with these words:
“If you took Madison and Milwaukee out of the state election formula, we would have a clear majority,” he said.

Now that, of course, is pure nonsense. It was word salad that went bad.
It'd be like someone calling Chicago sports talk radio and saying, 'If you took Brett Favre and Aaron Rodgers out of the record books, the Bears would have a clear majority' of wins between the two teams.
Or, ‘If you took Madison and Milwaukee out of the state population formula, Wisconsin would be without its two biggest revenue donors. And its biggest economic, cultural, financial, educational, commercial and transportation centers.’