If zelinski wanted to do the deal now it's too late. That deal is done. The next deal that will be offered may not be as favorable to Ukraine if zelensky is the negotiator.
Also keep in mind that zelinski did not have to come to the USA to sign that deal. He wanted to come to the USA for his own reasons. I don't think it was to simply sign the deal I think he came here to leverage his popularity for greater support from the American people. But things didn't work out as he hoped.