Why does Charlie lose his knowledge faster than he gained it?

In a letter to Dr. Strauss, Charlie, still highly intelligent, explains what his study of the data has revealed to him about his coming intelligence loss:

Artificially increased intelligence deteriorates at a rate of time directly proportional to the quantity of the increase.

What this means is that, because the quantity of intelligence Charlie has acquired is so vast, he will lose it more quickly. Presumably, if he had learned less and not become so intelligent, his rate of deterioration would have been slower. He is, however, the victim of his own success.
Through the pain Charlie undergoes—of tasting what it is like to achieve high intelligence and to function fully as an adult, and then to find he is going to lose it all and become mentally handicapped again—we come to understand the problem with rushing into human experiments too quickly, before enough groundwork has been laid.