Why can a Roth conversion affect credits and other tax items?

Because the taxable portion of a Roth conversion can raise income for the year. That can affect more than the tax on the converted IRA dollars. It may change credit phaseouts, the taxable portion of Social Security benefits, net investment income tax exposure, state tax, Medicare premium thresholds, or estimated tax requirements.

The clean EA workflow is to compare the return before and after the conversion. If a USD 35,000 conversion also reduces a credit or increases another income-based tax item, the effective cost of the conversion may be higher than the ordinary bracket alone suggests.