Primary reason not to shoot it down before then was that engaging at that altitude risked spreading debris at random over a huge chunk of potentially-populated US and Canada
But it's a balloon I don't see how it'd be that big a deal to pop it over the land, were they worried it'd be filled with anthrax or something?
What exactly do you think they would shoot it down with? You have to use cannon shells (which spread over a large area and potentially cause damage and casualties to civilians on the ground) or missiles (which explode, sending shrapnel in every direction, potentially causing casualties and damage to civilians on the ground). Not to mention that the thing had some pretty substantial electronics hanging from it, which have to go somewhere after being shot down. You
can not be certain where that debris will go - if there's even a single civilian in the potential damage area (which might not even be your own civilian - the potential damage area for big parts of the balloon's flight included southern Canada), you have to assume that civilian will get smoked. That's a reasonable risk to take against an attack aircraft, but this was a spy platform. Nothing it could hear and transmit before hitting the ocean (where you can put up a big "Nobody's allowed in here right now, if you get killed while trespassing it is your own damn fault" declaration) justifies even property damage.
A 20mm cannon shell or fragment of a continuous-rod warhead is far heavier than a penny, and absolutely will injure or kill you if it comes crashing down on you from 60,000 feet. If you look at WWII pictures of AA men or air wardens (the chaps who went around making sure blackout rules were enforced and helping people to bomb shelters), they all wear helmets. That's not to protect against bombs from the enemy - it is to protect against debris from friendly AA shells.