One of the points is that settlements systems are private permissioned blockchains, which don't take advantage of one of the main innovations of the public blockchains, i.e. decentralised trust. Interestingly I noticed that the recent Bank of England Real Time Gross Settlement Proof of Concept report[0] didn't even use the term "blockchain", preferring instead to use the term "distributed ledger technology" - I wonder if they are concerned about negative connotations the term "blockchain" might be starting to have.

[0] https://www.bankofengland.co.uk/-/media/boe/files/payments/r...