Flywheels are quite limited in their storage capabilities. Because there is a limit to their RPM before they tear themselves apart from centrifugal forces. They are really interesting for energy storage on the scale of seconds, to smooth out spikes over that range. Much less interesting for storage on the scale of hours to days. Where batteries and hydrogen become more interesting. Pumped hydro is also quite nice on those scales, but cannot be implemented everywhere.

Hydrogen is much less efficient that batteries, but much more scalable. That makes it attractive for taking in large amounts of over-produced electricity, and for use-cases where you need hydrogen anyway rather than meaning to turn the hydrogen back into electricity.