Looking on the internet deeply has found these results:

I don't know why. is the most popular phrase on the web.

We'll go out whether it stops raining. 10 results on the web	I don't know why. 19,070,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	Aug 16, 2014 ... SITE: http://danlimavideos.esy.es/ Musica: I Don't Know Why Artista: Moony Ano: 2008. Bibliografia: http://pt.wikipedia.org/wiki/Moony�... "I Don't Know Why (I Just Do)" is a 1931 popular song. The music was written by Fred E. Ahlert, the lyrics by Roy Turk. The song was published in 1931. songwriter Stevie Wonder, from the album For Once�... "I Don't Know Why" (sometimes listed as "I Don't Know Why I Love You") is a 1968 song by American singer I waited 'til I saw the sun. I don't know why I didn't come. I left you by the house of fun. I don't know why I didn't come. I don't know why I didn't come. When I saw�...

We'll go out whether it stops raining.

10 results on the web

I don't know why.

19,070,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

Aug 16, 2014 ... SITE: http://danlimavideos.esy.es/ Musica: I Don't Know Why Artista: Moony Ano: 2008. Bibliografia: http://pt.wikipedia.org/wiki/Moony�...
"I Don't Know Why (I Just Do)" is a 1931 popular song. The music was written by Fred E. Ahlert, the lyrics by Roy Turk. The song was published in 1931.
songwriter Stevie Wonder, from the album For Once�...
"I Don't Know Why" (sometimes listed as "I Don't Know Why I Love You") is a 1968 song by American singer
I waited 'til I saw the sun. I don't know why I didn't come. I left you by the house of fun. I don't know why I didn't come. I don't know why I didn't come. When I saw�...