zhī​suǒ​yǐ HSK7

reason why

之所以 zhī​suǒ​yǐ
reason why conjunction
the reason why (used after a noun or pronoun and before a predicate, typically followed by a clause explaining the cause)

The reason he was late was because of a traffic jam.

Frequency

Movie subtitles
17.3 per million appears in 8% of films
Written text
43.4 per million

Details

HSK 3.0
Level 7

Sources

Definitions
CC-CEDICT