The Vita is far from dead and buried, but nobody seems to have told Sony that.
Look, okay, no: the Vita is not home to massive triple-A releases. But it's really, really, really not dead or low on games. If you like 'em quirky, indie or even just Japanese, the Vita is just packed with content.
I guess I've answered my own headline question: E3 2015 is (mostly) about big triple-A blockbusters-in-potentia, and since the Vita doesn't have any of those to bookend a line-up full of little treasures, Sony decided it wasn't the time and the place.
Still, this excellent Vita sizzle reel has definitely been sent out to die. It's on the PlayStation Australia and New Zeland channels, but I wans't able to find it on any of Sony's other social media presences. Sadness.