The people who write slashfic tend not to watch W, but there's a little bit of it if you look in the right places. There's gobs and gobs of slashy fanart from the Japanese fans.
It's too easy, that's why!
Apparently, you have to nearly hate each other for people to slash a couple up. Or be a fountain of Angst with a forced "Forbidden Love" angle. With the hate thing added onto the pie for good measure still.
Seems like Shotaro and Philip aren't related enough or hate each other enough for slash fans to take notice.
.. . Do you smell something burning? And why do I see the shadow's of pitchforks on the wall?