A lot of financial transactions and central clearinghouses require participants to post collateral. For example if an insurance company enters into an interest rate swap with a bank, both sides will have to post some percent of the contract's notional value in escrow. This protects both sides from counterparty risk (i.e. what if the insurance company goes out of business and can't pay its side of the swap).
The collateral needs to take the form of low-risk, liquid securities. Usually government bonds. Bringing a big bag full of cash to a derivatives exchange is not accepted. If you're a big financial institution, you have no choice but to buy government bonds. Even if they're negative yielding.
Since 2008, there's been a massive increase in financial regulations. Policy-makers have desperately pushed to make banks and other financial institutions less risky. That mostly means much higher capital requirements and more central clearing. In turn that means the demand for holding high-quality government has exploded.