It seems easier, at first blush, to make an armoured car than it does to put a plane in the air, yet there were significant aerial engagements in WWI.
There were armoured cars used in the early stages of war and buses were used to move troops around. However there had been limited investment in armoured cars meaning they available only in small numbers as this was in the days before mass production the French had to empty the city of Paris of buses, taxis and even civilian cars to reinforce the front.
The other issue was that these were still basically civilian vehicles (in the case of the cars with armour plate bolted on top) with very poor off-road ability (narrow wheels ) and poor reliability. While the armoured vehicles offered protection from rifle and machine gun fire any artillery was death to them. To defeat the enemy if he advanced cross country you'd have to abandon the buses and engage on food, opening you to machine gun fire.
By the time better vehicles have been developed the stalemate had developed and the ground was impassable for wheeled transport. This was one of the reasons why tracked vehicles so interested the army.
The Brits had developed, or were in the process of developing a version of their Mk.V tank as a supply carrier, further developing it as a troop carrier with an artillery carrier as well. However, none were in service before the end of the war. http://en.wikipedia.org/wiki/Mark_IX_tank