I read that this balloon came in over Alaska and then Canada. At this time of year, there is a longer dark period there, it is often cloudy, and there are not a lot of people in the area, especially not a lot looking up with binoculars or telescopes like astronomers are thinking.
That said, we are now hearing that this balloon was known and tracked by the government, starting while it was in Alaska. So, the news media probably was not going to be informed until there was a substantial public observation, which seems to have occurred in Montana once the balloon got past the mountains.
There is now a briefing paper for Congresspeople (viewed only in a secured room at the Capitol) which talks about other, previous balloon incursions, and admits that there were at least 4 that got over parts of the U.S., including Hawaii and Florida, that were previously identified as "UFOs". Not to be confused with "alien space craft", it is just a recognition that something was reported without being identified. There is also a report that a similar balloon came down near Hawaii 4 months ago, which may be what got our government looking for other balloons, past and present.
It was interesting to learn that this balloon was inspected by U-2 aircraft, which are capable of looking at it from its own altitude. So, there is a lot more info about what this balloon looked like and was fitted with in the way of externally visible instruments and maneuvering apparatus. We in the public are not currently seeing any of that info, probably for security reasons, but I expect it will be released pretty soon. Once there is an admission that info exists, it usually means that at least some of it will be declassified.
Considering both the international and internal politics involved in this situation, I am not surprised that it took a well publicized observation by members of the public to make it public knowledge. In the (not recent) past, the U.S. has flown manned aircraft over both the USSR and China without telling the U.S. public. And, the Soviet government was also aware of the spy flights, but did not mention them publicly until they were able to stop them physically - which they demonstrated by shooting down the U-2 piloted by Gary Powers on 5/1/1960. So, there is the matter of political embarrassment, national security secrecy, and the "well, we are doing it too" issue that we may or may not know anything about in the public arena.
My concern is more about how the same technique could be used by rouge entities to deliver bombs. As Helio mentioned, Japan tries that in WWII, without much success. But, with the technology the Chinese demonstrated, it looks much more feasible to do it successfully, now. So, I am agreeing that we have a defense "gap" that needs closing in the high-altitude arena.