"Not least because..." - negation as a set phrase

Solution 1:

The complete phrase 'not least because' appears at least as early as 1801.

Much are we disposed to view it with complacency, on account of the subject, and as the first production occasioned by the memorable event, and last, though not least, because it is the effort of a female pen; ....

The British Critic, v 17-18, 1801, p 518.

I did not find the phrase significantly earlier than the turn of the 18th century. Notably, it was there part of the variable 'last not least' idiomatic phrase, which is considerably older — from before 1595; an attestation from 1536 in an 1888 publication,

...and last, not least, the very lamentable death of the good queen (Katharine), ....

Calendar of letters, despatches, and state papers, ... v.5 pt.2 1536-1538,

may have been, but is not likely to have been, the product of a heavy editorial pen.

Whether or not 'not least because' qualifies as a set phrase depends on the definition of 'set phrase' intended. See the ELU question "Is 'set phrase' a set phrase?" for some definitions of 'set phrase'. The now defunct Freebase noted succinctly and accurately that "There is no clear dividing line between a commonly used phrase and a set phrase."

Solution 2:

"not least because" OED (advanced search)

Here are the only seven entries that an OED advanced search provides--I have not followed up on the sources. This list demonstrates that the phrase has been around for a while but isn't a set piece, and in none of these quotations does it occur with "last but".

1. stick, v.1 View full entry eOE ...they have just to stick it out — not least because of fears that anti-sickness ...

2. matter, n.1 View full entry ?c1225 ... far more expensive full page, not least because it is set into news matter....

3. usage, n. View full entry c1325 ...bar during normal usage hours, not least because it will shield off some of t...

4. shopping, n.2 View full entry 1757 ...will never find a lower price, not least because it is commercially convenien...

5. Motswana, n. and adj. View full entry 1830 ...e for the individual Motswana, not least because of privileged access to the ...

6. clocking, n.2 View full entry 1872 ...mes rather an unreliable guide— not least because ‘clocking ’..is a far too common...

7. octet, n. View full entry 1879 ...e by Hausmusik is fascinating, not least because it is the first time I have ...