The reason for this is the fact that large businesses profit from being the middle-man here. Moving towards automated settlement on a blockchain reduces cost and increases efficiency,it's a win-win for everybody except the middle-man
Also, stock markets are heavily regulated. In a centralized system, it is relatively easy to enforce regulations. How do you do that in a blockchain based system?
The real issue is not the specific technology, it's that it's an ancient hodge podge.
T+2 is an atomic commitment algorithm that leaves enough time for humans to make phone calls and discuss the implications when there's going to be a failure to deliver or another anomaly from fraud or globally inconsistent state.
https://en.wikipedia.org/wiki/Depository_Trust_%26_Clearing_...