Why is the reaction slower as the concentration of the hydrochloric acid is watered down?

H.Cl (hydrochloric acid) exists as H+ and Cl- in solution. The H+ is responsible for the acidity and most of its reactivity. 2HCl + CaCO3 -> CaCl2 + CO2 + H20. By increasing the amount of water present you are decreasing the chances of HCl interacting with the CaCO3 as there will be less of the acid per unit volume.