How did English come to use a writing system which makes spelling it so hard?

Adding to the comment by John Lawler:

According to An Introduction to Language, 5th Edition (Victoria Fromkin & Robert Rodman, Harcourt Brace Jovanovich College Publishers, 1993):

"The spelling of most of the words in English today is based on the Late Middle English pronunciation (that used by Chaucer) and on the early forms of Modern English (used by Shakespeare).... When the printing press was introduced in the fifteenth century, not only were archaic pronunciations "frozen," but the spelling did not always represent even those pronunciations."

In short, spellings started to become standardized with the invention of the printing press, while pronunciation continued to change. The best example I remember from my Linguistic course was the word "night." The letters "gh" are silent now, but at the time standardized spellings were being established, they were pronounced. At that time, the word was actually pronounced closer to a throaty, German "Nacht."