Duration matching, I'd say. They want instruments that balance their outstanding obligations (ie pension liabilities thirty-fifty years out)

denlillaapan

I don't find the demand from pension funds very convincing.  Would the pensioners themselves have chosen the 100 year bond?  Seems like there could be a misalignment between the incentives of the fund managers and their beneficiaries.  Perhaps the pension fund managers are in bed with the finance bros at Google?

econ

These Bonds Are Ridiculous (WSJ, Spencer Jakab)

This.  

I don't find the demand from pension funds very convincing.  Would the pensioners themselves have chosen the 100 year bond?  Seems like there could be a misalignment between the incentives of the fund managers and their beneficiaries.  Perhaps the pension fund managers are in bed with the finance bros at Google?