A reporter asked them why they love China and their answer was because China has a long history. （误）

A reporter asked them why they love China and their answer was that China has a long history. （正）

究其原因，从根本上说，because 从句是不能充当表语的，它仍然是状语从句，因为主语必须是指“事件”的名词或代词，不可以是 answer。这里的 was = happened，answer 不可以 happened。

我们常说：This is because... ，这里的 this 指事件。

又如，The accicent was because the driver was too careless.

由于，was 的真正意思是 happened，because 从句实际是状语从句，不是表语从句了。