Can anyone walk me through why the US underperformed emerging and developed markets last year and what would cause it to do so going forward?

I'm thinking of changing my portfolio up. Currently 50/50 in SWPPX/JLGMX. Thinking of going something like this:

35% SWPPX (SP 500)

25% PEIFX (emerging markets)

25% VTMNX (developed markets)

15% VIEIX

Just primarily so I get some more exposure to emerging markets and developed ex-us. Maybe even just 40/30/30 in SWPPX/PEIFX/VTMNX.