The reason why… is that… (之所以…是因为…)

Structure Subject + 之所以 + result，是因为 + cause

之所以…是因为… (zhīsuǒyǐ… shì yīnwèi…) is "because" turned around: you state the RESULT first, then the CAUSE. 他之所以迟到，是因为堵车 = "the reason he was late is that there was a traffic jam". It highlights the result and then explains it. It’s the formal flip of 因为…所以….

Examples

他tā之zhī所suǒ以yǐ迟chí到dào，是shì因yīn为wèi堵dǔ车chē。
The reason he was late is that there was a traffic jam.
我wǒ之zhī所suǒ以yǐ这zhè么me做zuò，是shì因yīn为wèi没méi有yǒu别bié的de办bàn法fǎ。
The reason I do it this way is that there’s no other option.
之zhī所suǒ以yǐ选xuǎn这zhè里lǐ，是shì因yīn为wèi离lí家jiā近jìn。
The reason I chose this place is that it’s close to home.

Levels are indicative: they follow the standard Chinese Grammar Wiki (AllSet Learning) progression, not an official HSK 3.0 mapping. We never assign levels by eye.

This point and the other 370, with audio on every example, inside Wanyu.