Certainly both parties are currently captured by lobbyists for large corporations (minus, in theory, the progressive wing of the Democrats). The documentary "The Swamp" nicely explains how this system in Congress was created by Newt Gingrich in the '90s.

But this fact only pushes the question back a step since now the question is why the DNC allowed itself to sell out its progressive principles so easily. By surrendering to the top one percent, the Republicans are hardly renouncing their "principles" (i.e. their social Darwinism). The conservative Christians, of course, are selling out Jesus, but that's a much longer story, beginning in the fourth century CE.

Still, the Democrats became the progressive party after the Republicans turned into the racist and fundamentalist one. The Democrats brought in the New Deal, the Reagan coalition started to end it (Clinton finished it off), and the Democrats have refused to re-implement it despite public approval for progressive policies (for greater gun control, higher taxes on the wealthy, etc).

So the question is why the relatively recent Democrats sold out progressive values and ordinary Americans. My answer is that their culture is weakened by postmodern hyper-femininity and decadence which compel them to surrender before the monstrous masculinity of rural America and the Republican sociopaths.