"There are several factors driving investors into long-dated bonds. Falling rates and a wave of Americans refinancing home loans means mortgage-backed bonds get paid off quicker than expected. Institutions who owned those bonds -- banks, mortgage real-estate investment trusts and fund managers -- are pushed into buying longer-dated Treasuries or interest-rate swaps as the quickest and cheapest way to replace the disappearing income. Buying also comes from pension funds and insurers that sell annuities. When yields fall, their liabilities often grow faster than their assets. That can increase the so-called "duration gap" in their books, which is the shortfall between what they are going to earn on their assets and what they owe to pensioners. To close the gap, these businesses need more long-term assets."

The article argues that pension funds and insurers are driven to long-term bonds to close the "duration gap". Why would long-term bonds be the natural candidate to close the "duration gap"?