Even if you think that Trump did no crime with quid pro quo, it sets a VERY dangerous precedent. Why? Because asking a foreign government for dirt on your political rival is wrong because it gives that foreign country leverage over the US.
For instance, say that Xi wants to get Trump to lift the sanctions, so, he sets loose a team of investigators to see if Biden or his son did anything wrong there. If he was to find something, he could call up Trump and tell him that he's got some really good dirt on Biden, but in order for Xi to give it to Trump, Trump would have to lift the sanctions.
THAT is why it is being looked into as something illegal. It could feasibly give another country leverage over ours. That is also why the Founding Fathers said that allowing other countries to mess in our elections is unconstitutional.