From Don Melton's Mouth [0]:

    We built our own browser because we didn't want to depend on another company for a critical application.

    We built our own browser engine because we wanted to use the technology in more things than a browser.
    
    We built that engine small and fast because Bertrand Serlet would have shot me if I had done otherwise. :)

[0] http://news.ycombinator.com/item?id=5006704