Why is the enzymatic reaction rate highest at the beginning and lowest at the end?

When the substrate concentration is limiting, the initial velocity is highest.  As the substrate is being used up, the concentration of substrate drops, and so the reaction velocity.

When the substrate concentration is high, the enzyme is said to be saturated, and the reaction velocity is independent of substrate concentration.

Let's call the material the enzyme works on substance A, and call the material it turns into substance B.  The rate the enzyme converts A to B is highest at the beginning because at the beginning, the concentration of A is at its highest.  It will continually drop after that as it gets used up.  When its concentration is high, the enzyme molecules have a greater chance of running into particles of substance A (in just the right way) and being able to convert them.

At the end, there's nearly no A left, so the chance of the enzymes meeting up with it is low, and thr reaction slows to its lowest rate.

Simple explanation:
The enzymatic reaction it lowest at the end because all the enzymes are almost all used up.