Yeah, why cast an Oscar winning actress? Ridiculous.
Couple of reasons I can think of:
Money - The Oscar win sometimes means taking the opportunity to 'cash in' & earning a few bob off the back of said win. (Doesn't always happen though, I think it depends on the agent said actor/actress is under contract to)
Most Hollywood actors/actresses play at some point in their careers 'a defining role' for which they will always be remembered even if they win an Oscar for a lesser role or they do win.
Examples
Schwarzenegger = Terminator
H Ford = Solo/Jones/Deckard
Stallone = Rambo/Rocky
Downey = Iron man
Even though Natalie Portman won an Oscar over 10 after first starring in Star Wars, she will be remembered for the latter rather than the former. Julia Roberts = same for Pretty Woman. As stated earlier defining role & Oscar win sometimes happens. Silence of the Lambs prime example. Forrest Gump another.
My point is Larson will be remembered years later for this role rather than the Oscar win to Joe public.