I have heard conflicting things on this.
I think that they realized it was a giant spy balloon when it was over Montana. Then they could look back and see that NORAD had tracked it all the way back to Alaska.
And now they are going back and looking at other anomalous data in the past and have identified three previous incursions.
At least, that's what makes the most sense to me.
It's also possible that they did identify it off the coast of Alaska, but depending on its path, it may have very quickly moved into Canada before we had a chance to react. Some parts of Alaska are pretty darn thin and remote. And for whatever reason we kept quiet about it, maybe with the cooperation of the Canadians in the meantime.
Speculating, but the point is that even if we did identify it earlier, I can think of a thousand reasonable explanations other than apathy at the highest levels. Maybe Canada wanted to observe it and not shoot at it and didn't want it public. Or maybe we figured out how to send false signals and wanted to subversively feed the Chinese bogus info before we publicly "found it". Or many many other possibilities.