Why does Quicksilver Sash only build into one item?

See title. Quicksilver Sash only builds into Mercurial Scimitar, an item with a lot of wasted stats on AP Mages or Tanks. Mikael's Crucible doesn't see much play outside of AP-Heal Supports like Soraka. Here's a thought:

Make Mikael's Crucible incorporate QSS. It already gives most of the Quicksilver active anyway. If we make Crucible build out of Chalice of Harmony and Quicksilver Sash (Instead of Forbidden Idol), increase the Magic Resist to +60 (The total of (25) + (35)), and increase the total cost slightly (Since Sash = 1550g, and Cup = 900g, so total would be 2450g) to around 2800g, it might see a lot of play on super mana-hungry champions like Galio, as well as mana-heavy Tank/Initiators who often end up having to give up their blue-buff to their lanes.