Everyone here is saying there's not enough light. That's not actually true. The sun produces more than enough energy to grow all the plants. The issue is one of distribution. You have to distribute very particular wave lengths of light to the entirety of the structure and all the plants. This is a hard problem. Mirrors are inefficient. Fiber optics don't do exactly what you want, and plus, a lot of the light energy is unusable, because it's the wrong wavelength. Most modeling done in the space indicates that -- with current technology -- it's more efficient to capture the sunlight, convert it to electricity, and then use the electricity to produce light at the correct wavelengths.

As a whole, this means there's less energy available to irradiate the plants, but there's more energy in the wavelengths the plants can respond to, so it's more efficient.

I mean... think about it. Every time you look at a tree and see green, that is light that the plant has wasted. Hydroponic lighting is typically that deep purple, red, and blue, that plants can absorb. The goal is to make the plant leaves black, indicating that all light is being absorbed. Otherwise, you're just wasting light.