I don't quite understand why anyone would purchase negative interest-rate debt (might as well keep your money in cash at that rate), but hey, it's a pretty sweet deal for borrowers.
For private investors= Safety.
If you live in Greece and there is a X% chance your money will be worth 50% less, but if you use that money to buy German bonds there is a 100% chance your money will be worth 1% less... how high does X have to be for you to buy the German bond?
Or look at the USA in 2008. Stocks are crashing. People are talking about the banks going under. If your bank fails FDIC covers you to 250k, but it can take awhile for you to get your money. A short term treasury or a money market that invests in short term treasuries pays..... %&$^ who cares what it pays? You want safety!!!! You buy it no matter what and find out your yield to maturity is negative, but by then you don't care. You have safety, and paying 1% in interest to get safety is worth it.
I would rather have stocks and bonds, but some people get scared and run to safety.... and most of them don't even realize over a 30 year period cash is probably the most likely to be worth less.
As noted banks have their own reasons that are out of their control.