Rotational energy goes up worth both the square or the radius and the speed, so it is much better bang for your buck to have a the mass centered on an outer rim connected to a hub by spokes and spinning really fast rather than just something uniformly heavy. The mass close to the centre doesn’t do anything for you.
The heavier it is also requires bigger more expensive bearings and accompanying system to get the thing spinning in the first place.
I looked at a flywheel that spun at 12,000 rpm. It was going to be located in an underground vault in case it ever got off its pedestal bearings it wouldn’t mow people down.
Like anything that stores energy but doesn’t generate the economics aren’t great.