Although from what I understand on March 16th, 2018 the US will at least get same-day ACH. However, it will still be batch processed only two times each day--During banking hours no less! https://www.nacha.org/rules/same-day-ach-moving-payments-fas...
There is a NPR planet money on why the US has such an archaic system: https://www.npr.org/sections/money/2013/10/04/229224964/epis...
Their hypothesis? Unsurprisingly, perverse incentives. Wire transfers in the US, which usually carry a high fee ($30) are very lucrative for banks so they don't have much interest in eliminating the need for wire transfers at the moment, but it appeared that there was growing interest in moving to real-time eventually.