The phrase "not least because" seems to be everywhere in journalism and other writing, often as an understated negation meaning, in large part because.
Individual publications, and even some publishers, lack the resources to mount an all-out arms race against ad blockers, not least because they’re unlikely to attract engineering talent when there are so many startups and established tech companies plying potential hires with equity—or at least free meals.
MacMillan provides a phrase definition of not least:
used for emphasizing the importance of a particular aspect of a situation
I at first assumed based on an Ngram search that the phrase was a recent one, but I found one use from 1897:
The figures given above must be highly gratifying to the examining bodies, not least because they show that the present stage of musical education warrants further demands upon those who aspire to authoritative recognition.
That use seems casual and idiomatic, so I assume the phrase must go back even further than that.
Would this construction qualify as a set phrase?
Did this phrase exist significantly earlier than the turn of the century?