There seems to be a consensus that Real time Settlement or even real time Clearing might not be feasible, without major risks. Can someone explain what it took

There seems to be a consensus that Real time Settlement or even real time Clearing might not be feasible, without major risks. Can someone explain what it took to move from T+3 settlement to T+2? Why did they not go from T+3 to T+1? Can clearing be real-time (or near real-time) with settlement happening overnight? Or do they need to be in sync?