Microfilled composites cannot have a high filler load due to:
Filler content of microfilled composite  is 50-60% by weight. Because of the greater surface area per unit volume of these microfine particles they cannot be heavily filled.
Reference- Sturdevant 6th ed P:218
Small size of filler particles which decreases  the overall matrix
More surface area per unit volume
Low modulus of elasticity
Wear of opposing tooth structure caused by microfillers