First, reporters are incapable of discovering this information on their own. They just don't have the tools. Investigatory bodies do, and reporters would have to wait.
Secondly, there were multiple plausible ideas about what could have happened. The drum was beaten on only one of them. Trump got caught with his pants down because as with everything else he's ever been a part of, he was woefully unprepared and out of his depth.
Third, and tying in with the first point, the more important story to virtually everyone was not how the first people were infected, but what the response and fallout of the infections were.
So again, you had a bunch of clueless, embarrassed political partisans who got caught with their pants down, and their base ate up the blame-shifting because of their xenophobic tendencies.