He didn't have to. Because Zelinsky heard those words directly from Trump:
“I would like you to do us a favor though.”
shugerblog.com/2019/09/26/qui…
But he is clear that Giuliani ordered quid pro quo on behalf of Trump, serving Trump's "desires."
"Sondland said that I told him 'I want nothing.'"
That still doesn't change what Trump directly told Zelinsky he wanted in exchange for arms on his own (edited) transcript:
Trump: “I would like you to do us a favor though.”
Investigation.