Some of the largest custodial banks are interested in crypto custody but are hindered by the SEC rule SAB 121, which establishes accounting standards (Bloomberg)

Bloomberg:
Some of the largest custodial banks are interested in crypto custody but are hindered by the SEC rule SAB 121, which establishes accounting standards  —  - Coinbase, BitGo have been among the dominant service providers  — SEC rule seen making it impractical for banks to do custody