If you shut it down for too long and there is a lapse in reopening it, planes are grounded for an extra bit of time.

If you shut it down for too short and there is a lapse in extending the grounding, planes are getting shot out of the sky (or whatever threat it was).

edit: I would add that maybe there are forms for shutting down airspace of various specific time lengths and a convenient time for something of unknown duration would be 10 days. 10 days might also be enough time to be sure whatever resources need to be brought to bear on this are available where an hour or day might not be. Shut it down basically indefinitely, or at least long enough that the crew who handles this extraordinary situation will be on hand to turn it off.

Hoping it slips under other news like "Woah someone else should pay for this wall/bridge/investigation" so no one really notices it. To be fair, seems most things are about trying to direct the news somewhere else, most of the times being successful at that too.

NYT reports they're claiming it was about testing anti-drone tech at Fort Bliss.

> The brief shutdown was related to a test of new counter-drone technology by the military at nearby Fort Bliss Army base, according to a person briefed on the matter.

Hanlon’s Razor

"Never attribute to malice that which is adequately explained by stupidity."

Someone probably just screwed up