Certainly we let other countries know what we care about when we agree to give foreign aid. However, I think it is a bit unusual for a president to make that aid conditional on help making a case against electoral opponents. To the newly elected Ukranian president, it might have seemed even stranger. He might have seen this as the way Russia would behave and been surprised.
It is only a coincidence that the miscreants in this case include one of Trump’s opponents.
In fact, Creepy Joe Biden, who can’t even remember where he is half the time, is arguably among the easiest candidates for Trump to defeat.
So why should he want to knock him out of the race?
I believe that Trump’s motivation is to get to the bottom of the tampering with the 2016 election and the amassing of false documentation cobbled together by foreign powers to cripple his campaign and his administration.