Because they refused to say anything about it for weeks.
This isn't hard. You don't have to defend everything they do.

And you don't have to criticize everything they did. If you ask, "Why didn't Biden do that?", that's a really unjustified question when Biden did do that.

They didn't say anything about it immediately because they actually investigated it! They took the time to look into whether there actually was anything unusual happening there, and made sure that they were right when they said there wasn't anything. People were running around with all kinds of weird claims - even people on this board were taking very seriously the claims that these drones were somehow deliberately turning off their lights when anyone looked at them.

Trump just repeated what Biden had all his agencies say a month and a half ago. This is nothing more than people who didn't trust the government when they said it back then believing it when Trump says it. Which is fine, but it's not correct to claim that the Biden Administration didn't say it back then.