My understanding is that it’s basically because IRI is such a deep event. Most of the time all robots on each alliance are pretty close in ability, and it’s not unheard of for captains to actually act as the backup because their data shows all 3 picks to be higher performers. Given how good all the bots are 1-8,1-8,8-1 gives the top alliances a little more of an edge where lower seeded alliances might actually be the favorites with a serpentine.
This may or may not be the original or current intention, I have no reliable sources.
We started the 4 team alliance at IRI so that alliance captains would have input into their backup robot, and not be dependent on the luck of the draw. We also implemented the option for robots to be subbed in and out, so an alliance could change the mix depending on the match to be played. This also enabled a team to sit out a match for repairs, without being permanently replaced with a back up.
The alliance selection 1-8, 1-8, 8-1 was put I place to recognize / reward the top alliance with the first pick and 9th pick, to help them form a strong alliance after tough qualification matches. The 8-1 at the end helps balance that, by giving the number 8 alliance two back to back picks to make them a strong 4 robot alliance.