I suspect the problem there revolves around “how fast can you produce hydrogen?” Generally, oversupply events (and resulting negative prices) are brief. To take advantage of oversupply / negative prices, hydrogen production (or other energy storage) needs to be able to convert energy to storage very quickly.

I don’t know much about hydrogen plants, but I would guess that they aren’t super “peaky” — meaning they don’t have the capacity to ramp up and generate a lot of hydrogen quickly like they would need to really take advantage of the situation. If that is the case, hydrogen plants can still take advantage (simultaneously helping the grid and helping to stabilize prices) but not to the extent we would like.