Like equity, which is a long duration asset, these bonds
in the index are
noncallable with 25 - 30 years of maturity.
Back when dividend yields were higher, and corporate bond yields were higher, both absolute and relative yield managers flourished as interest rates and dividend yields crested
in the early 1980s, and the stocks paying high dividends got bid up as interest rates fell, much as the same thing happened to zero coupon and other
noncallable long duration bonds.