I can't answer for Boshea but, having a Scion deck myself, I rather think it has to do with the Mana base. Scion is a 5 color deck that is VERY color intensive in Red, and Black (at least). As a result EVERY SINGLE land you run that doesn't produce colored mana needs to have a really great and important effect to justify running it. I run
Boseiju because it is often the difference between crushing a control deck, and loosing badly.
Stripmine or
Wasteland is often an important role-player to get rid of problem lands like
Maze of Ith,
Gaea's Cradle, etc.
High Market and/ or
Miren, are important Sac outlets to protect your creatures from being Exiled or Scion from being tucked.
Volrath's Stronghold combined with
Sneak attack becomes essentially a second
Genesis and with either
Keiga, or
Yosei can lock an opponent out of the game very early.
Comparitively
Skarrg, is just not very impressive and in a deck where you REALLY need to cover your mana fixing, as much as possible, with your lands you really want to run the bare minimum colorless producers that you can while still covering the roles you need filled. Sure if get down
Prismatic Omen you are set, but from my experience that card doesn't tend to last in play for very long; you can't just rely on it to do all your color fixing for you.
