You spotted it!
It’s not flat, there’s a very small, shallow, polished depression in the middle that centres the ball in normal operation. The cage is normally quite loose. To prevent this being a problem it’s made for a material that has some limited self damping characteristics and is very light weighing 0.33g.
The brass had to be locally hardened to deal with the contact pressure from the silicon nitride ball, so using lead (or tin) would be possible but would leave it open to transmitting more transverse vibration.
Additional differences are that
1 the lower sound velocity in lead would be beneficial in increasing energy reflection.
2 the increased contact area at the lower interface would increase energy transmission.
Unfortunately I don’t have the necessary high speed laser interferometer to really analyse the system (I don’t think the accelerometers in my’phone would be sufficiently sensitive to measure the overall effect either).