Answer:
To compensate for the risk that they will receive less than promised if the firm defaults, investors demand a lower interest rate than the rate on U.S. Treasuries.
Step-by-step explanation:
Investors are risk averse, this means that they will always prefer those investments with lower risks. Since US treasuries are considered the safest investments, they are used to calculate the risk free rate.
When investors invest in other securities (not US government) they will always demand a higher return because a private entity or even a state or local government can default on a their debt. That difference between the return yielded by a US security and the return from any other investment is called the risk premium.