Why is the "Mainline" Church Liberal?

Here's one answer:

Lutheran theologian Richard John Neuhaus suggested that a loss of faith was the
main reason why so many Catholic and mainline Protestant leaders had turned
their churches into pulpits of the hard left. Not truly believing that the
Gospel was true, the leaders sought to make it socially useful.
http://www.davekopel.com/Religion/Evolving-Christian-Attitudes.pdf - p. 34

And that's the problem with a gospel that's not true: it has no real and lasting power. As Neuhaus explained, the leaders of the liberal churches tried to limit and control the gospel. They "sought to make it". . . into something it isn't.

That's not to say that there are no practical benefits of the TRUE gospel, but the pragmatic results of the gospel are not its main purpose. After all, they are results - the outworking of lives transformed. The "socially useful" part of the gospel is not its main goal.

Transformed lives, people changed - that's the goal. . . with the added benefit of socially useful (pragmatic) results.