のは〜だ details
Examples
it is X that — one element is singled out and stated at the end as the answer
彼に初めて会ったのは、去年の夏でした。
kare ni hajimete atta no wa, kyonen no natsu deshita.
It was the summer of last year that I first met him. neutral
今いちばん食べたいのは、やっぱりラーメンかな。
ima ichiban tabetai no wa, yappari raamen kana.
What I want to eat most right now is ramen, I think. casual
本日お集まりいただいたのは、来年度の方針をご説明するためです。
honjitsu o atsumari itadaita no wa, rainendo no houshin o go setsumei suru tame desu.
The reason we have asked you here today is to explain next year's policy. formal
返事が遅れたのは、メールに気づかなかったからです。
henji ga okureta no wa, meeru ni kizukanakatta kara desu.
The reason my reply was late is that I didn't notice the email. neutral
昨日行かなかったのは、単に忙しかったからだよ。
kinou ikanakatta no wa, tan ni isogashikatta kara da yo.
The only reason I didn't go yesterday is that I was busy. casual
Formation
普通形＋のは＋名詞だ（名詞・な形容詞は〜なのは）attaches to plain clause
Notes
a noun phrase, 〜こと, a な-adjective, or 〜から／〜ため. A bare verb cannot
close the sentence there.
Related patterns
Similar
Ordinary は sets up something already in play and comments on it; this frame manufactures the topic on the spot out of everything the speaker and listener already agree happened, so the only new information in the sentence is the phrase after は.
Lookalikes
Both put の in front of です and both explain, but the の lands in different places — んです leaves the sentence intact and adds background to all of it (電車が遅れたんです), while 〜のは〜です pulls one phrase out and answers with it (遅れたのは電車です).
Combines with
The の is that nominalizer with は on it — the clause has to become a noun before it can be the topic, which is why 〜のは takes the plain form inside it, not です.
With からです in the focus the frame becomes the standard way to answer なぜ in writing — 〜のは〜からです states the reason as the one thing at issue, where a bare から clause would just append it.