•  Interest on debt is TAX-deductible (hence the (1 - Tax Rate) multiplication in the WACC formula).
• Debt is senior to equity in a company's capital structure - debt holders would be paid FIRST in a liquidation or bankruptcy.
• Intuitively, interest rates on debt are usually lower than the Cost of Equity numbers you see (usually over 10%). As a result, the Cost of Debt portion of WACC will contribute less to the total figure than the Cost of Equity portion will.

 However, the above is true only to a CERTAIN point. Once a company's debt goes up high enough, the interest rate will rise dramatically to reflect the additional risk and so the Cost of Debt would start to INCREASE - if it gets high enough, it might become higher than Cost of Equity and additional debt would increase WACC.
It's a "U-shape" curve where debt decreases WACC to a point, then starts increasing it.

 However, the above is true only to a certain point. Once a company's debt goes up high enough, the interest rate will rise dramatically to reflect the additional risk and so the Cost of Debt would start to increase - if it gets high enough, it might become higher than Cost of Equity and additional debt would increase WACC.
It's a "U-shape" curve where debt decreases WACC to a point, then starts increasing it.