之所以是因为

wǒ zhī suǒ yǐ chí dào shì yīn wèi méi gǎn shàng gōng gòng qì chē

我之所以迟到，是因为没赶上公共汽车。

The reason why I'm late is that I missed the bus.

wǒ men zhī suǒ yǐ bù qù shì yīn wèi wǒ men fù dān bù qǐ

我们之所以不去，是因为我们负担不起。

We aren't going, for the simple reason that we can't afford it.