Well, because it was clearly bad. But with the latest patch Blizzard buffed Seasoned Marksmen, so, the build should become more viable. If it works well, I'll add a better variation of it :)

P.S. If you are playing Thrall right, you will never need Windshear. Plus envenom doesn't work against the meta (the Butcher outheals it, Samuro can just Mirror Image, others like Nova and Li-Ming die to the burst anyway).