Geez, what a completely moronic question!
You might just as well as "how come DNA didn't get discovered until the
20th century?" or "how come penecillin wasn't discovered until the
1930's?"
You must be retarded to ask such a question.
Evolution can be rather slow - happens over the geologic timescale
(thousands of millions of years); humans have trouble paying attention
to anything that takes more than a few months to years to happen.
Shit, what was I thinking?