Response to an emergency is restricted by one factor or the other, heavy traffic, lack of vehicles (which in developing countries may not be the case), lack of personnel, etc.
Depends whether the fire incident is situated in an urban or wildland landscape.
However in general, other factors may easily include:
weather conditions, direction of fire propagation and speed of fire front (including the effect of wind), accessibility of the terrain, selection of appropriate flame retardant means (this could be the case in industrial areas), time of the day (airplanes cannot be used after sunset) etc.
One of the issues in urban areas is the ability for firefighters to set up at the incident and access on-site firefighting equiment, For example, the on-site equipment might be not maintained, not visible, blocked or not appropriately designed for the fire at hand.