Some ideas, brainstorming here...

Tracking: satellites are incredibly easy to track once you know their orbital parameters. A balloon, far less so. You need a lot of active tracking.

Altitude: a balloon is much closer to the ground than a satellite. That allows a much smaller camera to take the same picture. It also could give you access to radio signals that might not be as easily monitored from space.

Harder to spot: being closer to the ground, it will be beyond the horizon until it's much closer.

And of course it's far, far cheaper. The Chinese space program is doing well, but it's still very expensive to build and launch satellites.

There are of course numerous downsides.