Originally, the radios were inside the cabinets on the ground. Power and data cables flow into the radios, and a broacast signal flows out to the antennas over coaxial cables. As soon is this happens, you have signal attenuation and an opportunity to introduce interference. An RRU lets you move the radios out of the cabinet and closer to the antenna, thus reducing the attenuation. The AIR antenna effectively builds the RRU into the antenna, making the effective distance almost zero.
Now an RRU can give you 98% of that benefit, if you mount the RRU right next to the antenna. Or you can mount the RRU further away from the antenna (maybe the structure can't support that much weight at that height), but you introduce more signal attenuation/interference the further from the antenna you move it. So you get some additional flexibility using that setup. But it does make the setup a little more complicated, and there may be some benefits in terms of total weight or wind loading by combining the RRU and antenna into a single unit. So there are pros and cons to doing AIR vs antenna and RRUs. But naturally if it wasn't available at the time and doesn't support CDMA then it wasn't an option for Sprint.