With a 10-year bond, the investor receives that interest rate over the whole 10 years. With a 6-month bond, the investor receives that interest rate only over these 6 months, and then has to find another bond to buy. If all available bonds then have a lower interest rate, the investor would have received more for the 10-year bond.
(What might be confusing at first is that interest rates are usually "annualized", that is, presented as if they were for a single whole year. You won't actually receive 2.49% of what you paid for the 6-month bond; you'll receive something like 1.24%, and you have to invest again for another 6 months to reach that 2.49%. If that investment is no longer available, you might not be able to do that.)
(What might be confusing at first is that interest rates are usually "annualized", that is, presented as if they were for a single whole year. You won't actually receive 2.49% of what you paid for the 6-month bond; you'll receive something like 1.24%, and you have to invest again for another 6 months to reach that 2.49%. If that investment is no longer available, you might not be able to do that.)