Because it's much easier to get a clear photo when the jet is moving relatively slowly at low altitude.

It's also easier to control an inherently unstable tailless delta-wing at low speed, with gear down and control surfaces up. Which is exactly what we see.

It's possible this was an attempt to make the videos possible, but it's also plausible that the videos were possible because this thing needs to fly low and slow to stay in the air in its present form.