Why are PIN junctions better for solar cells than PN junctions?
The PIN junction will have a wider depletion region which means it'll have a lower capacitance... so wouldn't PIN junctions be able to store less energy... right?

Storing energy capacitively is not important to a solar cell's operation.

Also, the electric field is also weaker in a PIN diode due the wider depletion region...

What's more important is that with a wider depletion region the photons have a longer path to travel in the region where they can be usefully captured, and a lower chance to exit out the far side without being absorbed.
The efficiency of the cell has little to do with the electric field strength or junction capacitance and very much to do with the fraction of the incident photons that are absorbed.