Question

Why does the rate of reaction decrease as the reaction proceeds?

Answer 1

A reaction happens when reagents collide with sufficient energy to cause a reaction.

The rate of reaction is the number of successful collisions per second.

As a reaction proceeds, the concentration of reagent decreases because…

It gets used up,
the volume of solvent remains the same (so less solute per unit volume),
the concentration of products increase (so more non reagent particles to physically hinder further reaction.
This makes it less likely that there will be as many fruitful collisions for more reaction. That is, the rate of collision between reagents decreases.

Therefore the rate of reaction decreases as the reaction proceeds.