Because, at the time he wrote it, it was a theroy. Darwin didn't have every answer at the time and when his theroy was published, it created quite an uproar - you should understand that part at least.

Without little more than common sense, true observational skills, and the knoweldge to put it all together, he wrote what can only be considered as one of the most game changing moments in history. He did this without miroscopes, computers or space age machines. In short, he was brilliant.

The other side of your coin has man was made from clay, a woman made from his rib and a bite from and apple screwed it up. Great story if you like that sort of thing but it hadly passes the smell test.