Mutual funds are taxed differently than bonds. When a bond generates income, you pay ordinary income tax. When a stock, mutual fund or ETF generates dividends, those get taxed at a lower "dividend income" tax rate. For the median taxpayer, it means paying 22% of the bond income but only 15% of the ETF / mutual fund dividends.
Another difference is long-term capital gains. Most of the benefit from a bond is the income. For a stock/ETF/mutual fund, it's mostly the growth - the rise in value. That rise in value is "capital gains", and you pay less tax on that (just like you pay less tax on dividends).
When you have stocks in taxable, you'll pay a lower rate on the dividends, and a lower rate for selling that growth. While you also pay a lower rate when you sell a bond fund, that's usually far less than the bond income. So actually stocks tend to be better in taxable.
Because bonds throw off more income, and it's taxed at a higher rate, putting bonds in a Traditional IRA makes more sense from a tax perspective.