In Opera aperta, Eco argued that literary texts are fields of meaning, rather than strings of meaning, that they are understood as open, internally dynamic and psychologically engaged fields. Those works of literature that limit potential understanding to a single, unequivocal line are the least rewarding, while those that are most open, most active between mind and society and line, are the most lively and best

Overidentification tends to be at the heart of the Canon Character Sue. It's hard not to do it, too, because it seems only logical to fill in unknown details of a canon character's personality with details of your own personality when the character is so similar to you in other ways. This is why, I suspect, so many of the male characters in the source material are "feminized" in fanfiction written by female authors.

One of these reasons is that Anakin is smack in the middle of a major romance (more specifically, a major heterosexual romance.  Not sure how much that matters, but accuracy and all).  No matter how badly that romance is handled (and it’s handled pretty darned badly), we can’t deny that it’s there, and as long as Anakin is professing (badly) that Padme is the center of his universe, it’s a bit harder to throw him into bed with someone else.  Now, you could point out that this hasn’t stopped LOTR writers from slashing Aragorn with 2/3 of the Fellowship, but I would point out that the courtship there is in the past (or in the books, in a glorified footnote), and is a fairly minor part of the story. 

The second apparent answer, and perhaps the reason for the lack of such stories, is the lack of any “vibe” between Obi-Wan and Anakin.

Which brings us to the most often-cited reason: Anakin’s lack of appeal.

In LFN fanfic, the justification I have heard is 'what would it take to break Michael?' Michael, you see, is the top-op, superspy boy. Always in control, his emotions submerged behind a blank mask. And I understand that hurt/comfort is a common thread in many fandoms. It is the gratuitousness of the violence that bothers me. It’s as if some authors enjoy hurting Michael; please excuse the vulgarity, some actually get off on the idea of destroying this proud, difficult man/character.

The foremost condition that I can think of is that there must be two attractive characters of the same sex that people would like to fantasize over.

Another necessary condition is that the two candidates for slash must not be unredeemably heterosexual.  That rules out shows such as Remington Steele, The Scarecrow and Mrs. King, and Moonlighting, in which the men with looks to die for spent season after season chasing the female lead, frequently culminating in wedding episodes of great pomp and circumstance, albeit with a total lack of credible plot.