Concentration of the reactants decreases over time as they are used up in the reaction. This means that there are fewer collisions between the reactant particles per unit time, leading to a decrease in the rate of reaction. Additionally, as the reaction progresses, the concentration of the product molecules increases, leading to an increase in the likelihood of the reverse reaction (i.e., CaCl2 + H2O + CO2 → CaCO3 + 2HCl) occurring. This also contributes to a decrease in the rate of the forward reaction over time.