Why do you think the U.S. Government is using two different types of logins? Login.gov is operated by the U.S. Government and ID.Me is a private entity that is separate from the government.

For Social Security you use Login.gov unless you live outside the U.S. or don't have a social security number.

For the U.S. Treasury where individuals can buy Savings Bonds, Treasury Bills, Notes, etc. they will be forcing you to use ID.Me.

I would be a lot more comfortable if the U.S. Treasury was using Login.gov.