Reading James Shapiro's Contested Will: Who Wrote Shakespeare, given to me by a friend retiring from the Eng. dept., puts some things into focus about hypercanonical authors like Lorca. Of course I'm reading it, selfishly, for what it can tell me about my own project.
The first factor is the deification of Shakespeare. Once bardalotry sets it, then the stakes get very high, and the need arises for an extraordinary person to have written such an extraordinary work.
A second mistaken assumption is that Shakespeare was writing out of his own personal experience, his feelings and beliefs and first-hand knowledge of things, that his work is basically autobiographical. For Shapiro, this is a modern assumption (rising out of romanticism, really), that all literature is autobiography.
Thirdly, there is cryptology: the belief in secret codes.
Now it might be true that modern beliefs about authorship are not valid for Shakespeare, but are for Lorca, who after all is a modern figure. In other words, I am fighting a losing battle if I try to contest the biographical dominance in the construction of the author figure. Deification, though, is the first step in investing so much in the authorial subject.