why (Calif.) apricot & peach jam will not set? For years, both my mother's and my own apricot and peach jams would never set up. The pectin package warned us that it might take up to 2 weeks to set (WHY??), and we always waited and checked, but it didn't happen then either.  Plum and blackberry jams always came out perfect every time.

Other acquaintances in the area (SF bay area) reported the same thing. I think I have a fix - just put in more lemon juice, like double (or even more), what the recipe calls for.  This has worked well for me a few batches now, but my question is why?

Why would the pectin company put a recipe in their box which doesn't work for so many people?  Are we all doing something wrong?  Or is there something about California fruit - or California lemons? - that is different from the place these recipes were written for? ... Or something about our atmosphere??  And why do these recipes mention that apricot/peach, specifically, might take up to 2 weeks to set.
Pectin needs acid to work. Fruit never has the same amount of acid. Not only do blackberries have much more acid than apricots, but two different cultivars of apricots and even two apricots of the same cultivar but grown in different conditions will have a large difference in the amount of acid. Ditto for lemons. 

Your fix proves that this is the reason - once you added more acid (in the form of lemon juice), the problem disappeared. So, the reason "why" is simply "you didn't have enough acid in your jam". 

The question of "why would the company publish a recipe" - it publishes a recipe which works on average, but it can't make a recipe which works for all fruit grown under all conditions. I can't tell you why your fruit is lower in acid than average (although the amount of sunny days during ripening is probably involved) but it seems that it is.