Generally speaking (all-AD teams aside), armor auras tend to be much harder to balance than magic resist auras. The main source of physical damage is ADCs or assassins, both of whom go for a single target; magic damage, on the other hand, tends to splash all over your team. Compare Caitlyn to Syndra, for example. With that being so, something like Aegis of the Legion helps your whole team at once, so you get the full value of the MR; with armor, only one or two people on your team are taking large amounts of physical damage at a time. With that being so, at any particular value, the armor given by the aura would either be
- Too low for it to provide a noticeable gold value, since only 2/5 of your team is using it at a time, as opposed to 5/5 with Aegis.
OR
- Too high, rendering it utterly oppressive against AD-heavy teams.
Balancing it for an average situation (one ADC and maybe one other significant physical damage threat) would make it obscenely powerful against AD-based teams, far beyond the already fantastic Aegis of the Legion, and balancing it against 4 or 5 AD teams would make it incredibly overcosted for any other scenario, rendering it a VERY niche item.