Changes to capital requirements for insurers and pension funds, for example, have been specifically designed to encourage them to match the duration of their assets and liabilities, 4/9  BIS central bankers' speeches  raising demand for long-dated sovereign issues. The new Liquidity Coverage Ratio induces banks to increase holdings of high quality liquid assets, of which government bonds are major part.
And the shift of a large number of OTC derivatives transactions to central counterparties (CCPs) elevates demand for safe assets for use in initial margin and guarantee funds. Second, safe assets act as a means of exchange, especially for the non-bank financial system which cannot settle claims with central bank money.