"What/why is the reason we have become so liberal...?"

Well, "so" liberal is a matter of opinion, depending on where you are on the liberal-to-conservative scale to begin with. The fact that America's become liberal isn't controversial, and you've asked the interesting question.

I find that our political history starts to make some sense if you start in the late 1920s, and then compare where we are today with then. That period of time is important for many reasons--the stock market crash, start of the Great Depression, soon to be followed by the New Deal...and most important of all, the birth of the American Conservative movement. You can track their growth across the next 90 years as a backlash against Liberalism, beginning with the liberal reforms of the New Deal. FDR introduced a strong dose of liberalism in the form of emergency measures and structural reforms (e.g. banking and the stock market), and when they all worked, the people approved and approved of FDR continuing along that track.

I think I can make a strong case that that track has been stunningly successful, and that the answer to your question is simply "it works".

Conservatives look at the trend to the left and predict dire consequences, that it'll reduce America to the status of North Korea; and I simply have to point out that, nunh-uh, it didn't happen that way, those liberal reforms ushered in America's true golden age of prosperity and global influence. America took off like a rocket once the New Deal got us out of the pit.

And I can point out the uncanny fact revealed by statistical analysis of the terms of Democratic and Republican Presidents, respectively, that all Americans do better economically under Democratic Presidents than they do under Republicans (see "Unequal Democracy" by Larry Bartels). Everyone's income, at all levels, rises by about 5% under Democratic Presidents since WWII (except the hapless Jimmy Carter); while under Republican Presidents, the average overall growth is about half that, and it's badly skewed: The rich get richer and the poor get poorer. I'm serious: Under Republicans, the poorest 20% actually do get poorer, and the top 5% gets richer. (It's a hoot to listen to Conservatives complain that Democrats want to redistribute the wealth.)

You can point to solid material gains under liberal policy, and you can also point to intangible yet meaningful improvements, like equality for women and minorities, and social safety nets that insulate people from the joys of the "creative destruction" inherent in free-market capitalism.

We'd have none of those things if we'd been governed by Conservatives for the last 90 years.

I rest my case.