独立主格结构,being 和 were 的区别在哪?

There being no buses,they had to walk home.为什么用being?非谓语形式吗?
Because there were no buses,they had to walk home.为什么用were?
There being no buses,they had to walk home.=Because there were no buses,they had to walk home.
There being no buses是非谓语形式.没有连词,用逗号与主句隔开.而Because there were no buses有连词Because连接一个原因状语从句.