Ive seen its use increase over the last few years. Mostly in online articles, as a "catchy" / click baity title , in the form:
"Here's why $thing" ; or
"$statement!. Heres why"
Many Examples: https://www.google.com.au/search?q=heres%20why
I've found this , which maybe a partial answer https://www.quora.com/Is-the-sentence-Here-is-the-reason-whygrammatically-correct
it has a more verbose sentence, but really, just tying to find out if the "heres why" has a name, or how it is defined in the language, grammatically?