>

Because they knew it was there.
It made it all the way to Montana unnoticed (from China) and it only was spotted because it was near a sensitive facility. 
I'd also really be interested in evidence that it was photographable with a cell phone at the height it operated at. Once the Chinese government was contacted telling them we knew about it I'm pretty sure they changed it's altitude to comply with our demands regarding it.
You can observe a lot just by looking - Yogi Berra. >

“Nobody goes there anymore, it’s too crowded.”

other balloons probably >

Republicans treat national security as a joke, and play politics with it. America will fall because of enemies from within. >

My little rural town is going nuts on Facebook taking pictures of the moon during the day and claiming it’s not the moon because of this balloon. Spoiler alert, they are looking at the moon. What did all the morons do before social media? >

Improved surveillance is great, but people were taking pictures of it with their phones.  It was, like, right there.