I think the US attracts traders from around the world but not enough because of EST hours. Open it up 24h and now you may have a trader in Berlin, Tokyo or Singapore ready to trade in his/her time zone.
You have that with the ES and with US treasuries. How liquid are they off hours?
One of the most liquid equity spreads is probably spx/nky and spx/eurostoxx yet I bet volumes in the ES is tiny during the off hours.