The grand Oxford conspiracy theory (which in its wilder variations has the Earl as both son and lover of Queen Elizabeth, as well as being the author of the works of Shakespeare, Marlowe and many more) then happened to hit the Zeitgeist of the last few decades, when we have learned that governments often do lie to us about more important issues than who wrote a play, and questioning received wisdom has become habitual.
Finally, Shapiro points out that Shakespeare's claim to sole authorship of all the plays is no longer accepted by mainstream scholars, in that several of the plays are in fact collaborations (with Fletcher, Middleton, Wilkins and Peele; and he omits Kyd and Edward III). The idea that even a small part of Shakespeare might not be by Shakespeare was heretical until surprisingly recently. But real research, unlike Oxfordianism or Baconianism, moves on.
A good book to read as I crystallize my own biographical endeavours.