Buffett's general MO is to find distressed or other undervalued companies and offer a large chunk of money, negotiating very favorable terms.

Take their deal with Bank of America, wherein they received preferred stock which paid 6% annual dividend (so they end up higher up the capital structure than the common) as well as a dilutive warrant to purchase 700M shares of common stock at a strike price of about $7 at any time until 2021 (currently at 29.49)

The article answers why the deal went sour:

> Coming so soon after that cash infusion, Buffett’s attempt to take a stake in Uber while it was on the rocks may have been too late to squeeze favorable terms from the company.

Buffett probably couldn't get the type of deal he wanted, mostly because they were able to secure cash independently.