Because the only reason to accept a low interest rate is to minimize risk. If that's what you want, you should put your money in a money market fund, which has essentially zero risk.
In a bond fund, if interest rates go up, you lose money. So the best time to buy them are when interest rates are at their highest point. If they stay the same, you make a high interest rate, and if they go down, you get capital gains, which will be even more.
Conversely the worst time to buy bond funds is when they are at historical lows, like now, for the same reasons in reverse.
Michael