之所以 … 是因为 … (effect before cause)

The real reason

• A journalist asks a famous chef why her restaurant has stayed popular for thirty years.
• we / us我们wǒ menthe reason why之所以zhī suǒ yǐpopular受欢迎shòu huān yíng，is是shìbecause因为yīn wèiall along一直yì zhíto use / with用yòngmost最zuìfresh (experience, food etc)新鲜xīn xiān(modifier / structural particle)的de(raw) material材料cái liào。 — "The reason we're popular is that we've always used the freshest ingredients."

the reason why之所以zhī suǒ yǐ + result，is是shìbecause因为yīn wèi + cause

• the reason why之所以zhī suǒ yǐ A，is是shìbecause因为yīn wèi B = "the reason (why) A is that B": the known result comes first, the cause second.
• the reason why之所以zhī suǒ yǐ follows the subject: he他tāthe reason why之所以zhī suǒ yǐwas late迟到chí dào，is是shìbecause因为yīn wèi….

he他tāthe reason why之所以zhī suǒ yǐwas late迟到chí dào，is是shìbecause因为yīn wèitraffic jam堵车dǔ chē。

Explaining in essays and speeches

• I我wǒthe reason why之所以zhī suǒ yǐchose选择xuǎn zéthis这个zhè gemajor专业zhuān yè，is是shìbecause因为yīn wèiI我wǒto / towards / correct对duìmedicine医学yī xuédegree / adjective linker很hěninterested感兴趣gǎn xìng qù。 — "I chose this major because I'm very interested in medicine."
• The pattern sounds considered and formal; it suits explanations and written arguments.

I我wǒthe reason之所以zhī suǒ yǐchose选择xuǎn zéthis这个zhè gemajor专业zhuān yè，is是shìbecause因为yīn wèiinterested感兴趣gǎn xìng qù。

Result first, cause second

Versus because因为yīn wèi…so所以suǒ yǐ…

• because因为yīn wèi A，so所以suǒ yǐ B: cause first, then result — the everyday order.
• the reason why之所以zhī suǒ yǐ B，is是shìbecause因为yīn wèi A: result first, as something already known, then the explanation.

because因为yīn wèitraffic jam堵车dǔ chē，so所以suǒ yǐhe他tāwas late迟到chí dào了le。

Recap

Read the full reference: HSK 5 grammar — Cause & effect statements.