The short answer: Inflation and Interest Rates
Both of these being very low is great for bond values and the value of their fixed returns. Inflation obviously erodes the fixed value of bond payments. Interest rates being low provide fewer alternatives for "safe" returns, and thus increase the resale value of bonds. If you think they are both going to stay low, then go for it. If, like most folks, you believe these are both on their way up over the next few years, you don't want all of your eggs in the bond basket. I will eat my hat if the next 10 years for those funds ends up as rosy as the last 10, when compared to the S&P.