Ivory marked by the Royal African Company. International Slavery Museum, Liverpool. Wikimedia Commons.
In 1999 Congress tore down the legal wall between ordinary banks and investment banks.
If money is tight, your checking account can now sit inside a company that also trades for its own profit.
If you are in the middle, the sales pitch is one firm for the mortgage, the card, and the brokerage.
If you have savings, the combination is convenient until the trading side endangers the deposit side.
The Gramm-Leach-Bliley Act of 1999 repealed the Glass-Steagall separation of commercial and investment banking.
Citigroup had already jumped the wall and then gotten the law changed to fit.
Whether the repeal caused the 2008 crisis is still disputed, and the combination itself is not.
Source: Gramm-Leach-Bliley Act, November 1999.