It is increasing. At the moment it is slowed because for the longest time we were fine with just hydro, pumped hydro, gas peaker plants and the natural inertia of the power plants' turbines and generators. Now demand is big enough that even lithium-ion is deployed for grid storage, despite lithium-ion being optimized for the opposite use case (light portable power storage). Lots of options that are more optimized for grid storage are in various stages of development, but it takes time for them to be brought to maturity and for operators to gain experience and confidence with them.