In dust explosions, why is a certain concentration of dust required?

Dust Explosions is characterised with a Pentagon which includes a combination of Fuel, Oxidant, Mixing/ Dispersion, Ignition Source and Confinement as shown in the image below:

Fuel concentration in the mixture of fuel and oxidant (aka. the flammable atmosphere) has to be optimum and within the flammability range. If the concentration is too high (atmosphere is too rich), amount of oxidant will be lower than the required amount. If the concentration is too low (atmosphere is too lean), amount of fuel will be to less. The ranges in normal terminology are called as Upper Flammability Limit (UFL) and Lower Flammability Limit (LFL) respectively. For the dust flammable atmosphere, the lower flammability limit is called as Minimum Explosible Concentration measured in terms of g/m3. If the concentration of fuel in the flammable atmosphere is too low, there is no possibility of fire or an explosion.