There is hard data that shows that a centrist Democrat would be a losing candidate
Now, why hasn’t the Democratic Party heeded Piketty’s warning? I think you already know why. To quote Upton Sinclair: “It is difficult to get a man to understand something, when his salary depends upon his not understanding it.” The donor base of the Democratic Party consists of a lot of pretty rich people who prefer the Democratic Party to be left on social issues but right on economic issues. The party elite see these wealthy folks as part of the party, and don’t want to nominate a candidate who accurately sees them as class enemies. I wonder sometimes if there are Democratic Party eggheads working for the DNC who are aware of Piketty’s prophecy but are not willing to risk evolving the party, lest they lose their benefactors.