Please someone correct me if I'm wrong here. There's only two reasons I can see why people would buy this.

Firstly, if you are buying in different currency to your own. If someone feels (for example) EUR is going to massively outperform USD over the next 30 years, then the small loss in bond value is more than made up by the increase in exchange rate. You can use these bonds to essentially trade currencies.

Secondly, if they expect the markets in general to do terribly over the next 30 years. If you think there is going to be a massive economic recession over the coming years, then holding a bond which is guaranteed to only decrease by a small amount might be better than taking the risk against a potentially much larger decrease in other markets.

The second is worrying, because it suggests that both the government which is offering these bonds and the large banks and corporations which are buying them are bracing for the worst. The recession which we are all expecting could be worse than imagined.