Why is penicillin not as effective as it was fifty years ago?

This isn't entirely true. Penicillin is still as effective against the bacteria that it combated 50 years ago, but new strains of penicillin resistant bacteria have evolved since 50 years ago. This means that penicillin is less effective against a larger percentage of bacteria than it was 50 years ago, solely because of the new, anti-biotic resistant strains.

For more information, search Natural Selection.