Missouri taxes ordinary income at 2%–4.95%. Roth conversions are treated as ordinary income, so a large conversion adds to your MO state tax bill in the year you convert. At the top state rate of 5.0%, a $250K conversion costs $12,375 in state tax alone.
Missouri taxes traditional IRA and 401k withdrawals as ordinary income. This makes Roth conversions more attractive: pay 5.0% now and never pay state tax on that money again, including decades of tax-free growth.
Missouri also taxes Social Security benefits. High provisional income from large traditional IRA withdrawals in retirement can trigger state tax on Social Security. Converting to Roth now reduces future RMDs, which lowers provisional income.