Why is Preaching the Word 1 Kings not in the Preaching the Word set?
https://www.logos.com/product/167610/1-kings-power-politics-and-the-hope-of-the-world
https://www.logos.com/product/119851/preaching-the-word-commentary-series
They do this all the time. They have a set and then as new volumes come out they keep them separate until they make a new set. It can take quite a while sometimes to get the new books in a new set. Look at the tyndale commentaries for instance... seems like there is always a new volume coming out.
In my experience, it is always better to wait for them to be put in a set... bigger discount usually.