I can't speak to anyone else's reasons, but for me it's (1) maximizing crit/det, (2) Fey Wind takes care of a lot of the benefits of high SS (half the time, anyhow), (3) BotD and Fey Wind have this synergy where they're both 60s roughly synced and you end up with a slightly higher number of half-rotations/minute overall, and that's what directly drives Geirskoguls / minute.

Even running 579 SS last tier, I consistently ran into what Thendiel calls "the dead zone" in his write-up because I was getting a GCD / minute rate closer to like 675 SS.