Why? It's a good question, it's not something that economists seem to understand very well.

至于原因是什么，的确值得研究，即使是经济学家，也并不见得完全明白。

斯坦福公开课 - 经济学课程节选

It's one of the questions of economics -what determines the term structure?

经济学中的一个老问题是,期限结构是由什么决定的呢

耶鲁公开课 - 金融市场课程节选