Because it affirms the malfeasance narrative that the coronavirus is this super scary disease on par with Ebola if it was easily transmitted. The MSM and elites do not care that death rate have significantly dropped, that the public is building up resistance to it, or that people who die from it are often compounded with other co-morbidity. This was the moment they were waiting for, the perfect occasion to bring the coronavirus back to the forefront. If Trump ends up being hospitalized due to this, they will run with "TRUMP IS SICK" until Election Day happens. The Democrats are relying on the most negatively energized base in history to throw Trump out and if that works, the lock downs will be repeated in the near future. They are already pushing for climate lock down and you can bet even if they suddenly "re-open" after Biden gets elected that the DNC will plan another lock down when the opportunity presents themselves.
We'll have to wait and see. If Trump makes a quick recovery, it can damage the COVID-19 narrative much like how a ACB confirmation would demoralize potential voters on the Supreme Court issue. But right now, we don't know what's going to happen.
Trump is also running a difficult re-election campaign where anything and everything can be used against him, especially his response to the COVID-19 lockdown. Boris was not when he caught it. I'd like to believe the public would be more sympathetic to Trump but I will believe it when I see it. Trump once again has to exhibit strength in order to come out with a fighting shot at winning this election. That's one of the reasons why Trump won the first debate against Biden, because he showed strength while Biden showed weakness.