Why can only earned income be contributed to an IRA?

It has little to do with rich people deferring unearned income. It is mainly to allow earners without an employer supplied pension to have their own tax free pension. That's why the IRS deduction is eliminated if you have an existing employer pension and make above a low level of earned income. Single, I couldn't contribute with W2 income above $66,000. And that's not rich.