Because, the powers that be, a.k.a. corrupt Biden family, sold to chinese commies, do not want what this balloon was all about, because their chinese overlords want to keep it hidden. After the payload of spy data was delivered home, i.e., china, the vehicle is destroyed. Otherwise, capturing it and reverse engineering what it was doing in US airspace, and what data it collected as it traversed the whole fucking country, would be very useful. Don't you think?