What causes the concentration of reactants to affect the reaction rate?
Higher concentrations of reactants = More collisions between molecules = More possible reactions between the molecules = Higher reaction rate
For there to be a reaction, 3 criteria must be fulfilled:
1) Collision between molecules
2) Molecules must collide with sufficient energy
3) Molecules must collide with proper orientation
If you have more molecules, which would mean a higher concentration of reactants, the probability of having collisions are higher. This would mean that a reaction would be more likely to occur, hence affecting the reaction rate.