There is a reason - persistent rumours.
If he truly thinks that way, why shouldn't he be allowed to check wherever it's true?Trump doesn't have the right to send his personal lawyer "fact finding" in a foreign country demanding their cooperation in exchange for the support they need to continue existing when half their country is facing pro-russia movements for already disproven conspiracies regarding his political rivals because in addition to Biden he seems to also think he's going to find Hillarys emails in Ukraine.
There also was no explicit "exchange" in that call.
And as far as "implicit" exchange goes, everyone in Ukraine also understood that paying Biden's son and listening to his other demands is one of necessary steps to get those billions of aid.