Why did Trump need a statement from Ukraine and not an actual investigation into the Bidens?

As President Trump’s most ardent defenders have reiterated in the weeks following the whistleblower frenzy, politicians initiate quid pro quos with foreign governments all the time. Trump may have threatened to withhold $400 million in congressionally approved aid to Ukraine, his surrogates argue, but as vice president, Joe Biden threatened to withhold a whopping $1 billion.

What matters in both cases were the stakes. Biden’s quid pro quo was a service to the global community, which begged the United States to oust corrupt Ukrainian prosecutor Viktor Shokin. It was an act to patriotically benefit the American people. Trump’s motive is the core of the current question rattling the country.