英语关于since用法的一个问题!

题目

英语关于since用法的一个问题!
有一个句子是：
Since the government was bankrupt,many of the soldiers who were sent to quell the riots had not been paid in months.
为什么要用“had been”这个过去完成时而不用现在完成时?

答案

因为这件事发生在过去啊!