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Enzymes are found in living organisms, and also behave like living beings.

When there are more of them, under normal conditions, they are more productive, like a larger group of people working on a project.

When the temperature is manipulated, the enzymes react much like humans and animals do. When it is too hot, they become lazy and slow down, and when it is too cold they similarly slow down. But if a certain amount of heat is added to the enzyme, a gradual increase in its rate will occur until it reaches its limit of reacting under high temperatures.

If you change the pH, you are affecting the "body" of the enzyme and its ability to work normally. An enzyme at a neutral pH is able to function most efficiently. As with the bell curve of the temperature's effect on enzyme reactions, before reaching the enzyme's limit, its reaction rate will increase with an increase of pH.