Why build a single damage type army in Game of Thrones: Dragonfire?

Because stat debuffs help every dragon dealing the same damage type. A single-type army turns each debuff into a team multiplier.

A concrete example with a fire army: your Jagadrix lands −Initiative on the enemy. Since Initiative reduces fire, all your fire dragons hit harder off that one debuff. If your team were half fire and half physical, that −Initiative would only help half the army.

The 4 single-type archetypes:

• FIRE: Seasmoke or Syrax at the Center, Caraxes / Shadowsong / Tashix, with −Initiative from Jagadrix or Crimson.
• PHYSICAL: Rhysarion at the Center, Daemoros / Thunderstrike, plus a Taunt tank.
• TACTICAL: Feskar or Kalspire, amplified by Zivern and Antares (more tactical damage taken).
• CONTROL / PANIC: Kalspire and Shadowrend apply Panic, which feeds the fire engine.

The rule: one dominant type plus the debuffs that serve it. Not an assortment.