Because it's been the only game in town since the financial crash people have forgotten that the US markets can do worse than the rest of the world, it looks like the us dollar is going to get weaker over the next few years which benefits emerging markets and commodity producing countries. Even the msci world index is heavily weighted to us markets now much more than a decade ago. You nearly need to be looking at etfs now that are underweight the US yet everyone seems to be focusing solely on the S&P 500 obviously only looking at its performance over the last decade