When did it become "mainstream" for women to shave or wax the pubic area?
I used to be a gynae nurse, and 20 years ago, no one did it. I can remember seeing maybe 1 or 2 a year (and seeing the pubic region of an average 20 women a day.) It was so rare, it would probably be remarked on.
I've had conversations with my friend who's still a gynae nurse, and she says she first started to see a few in the mid to late 90's, increasing steadily in frequency until the balance tipped towards more without than with by about 10 years ago and it was a higher proportion amongst under 30's than over 30's.
Where did it come from? Definitely from the fashion for waxing in porn. It wasn't to eliminate crabs, but to make it easier to see "what was going on" in close up shots of the female genitalia. If you note the dates, this corresponds with the beginnings of porn being available widely online, and a few years after the practice became common in porn.
A guy might have seen an actor with shaved genitals in a porn film, associated sexual pleasure with the imagery, then perhaps suggested it to his girlfriend. At the same time, salons started providing the service to cater for what started to be a demand for the practice. Now, Brazilians and Hollywoods are advertised on sandwich boards and newspapers, just like mobile phone deals or the dish of the day at a pub. It's become very normalised and now become almost de rigueur. One can argue it's "just a choice," but only in so much as choosing not to do it means you are now going very much against the "norm," and probably will have to defend yourself against insinuations that you are dirty, smelly, unsexy or unattractive if you don't.
I think it's interesting that the practice of vaginal douching never really caught on in the UK, but it's been quite popular in the US at least since the 1950's. I think folks will find these adverts make them 
(and Lysol is what my mum used to scrub the floors with - similiar to Jeyes Fluid here.) this, this and more recently this.
Douches are marketed on the basis that they make you "feel cleaner," although in reality, they can damage the flora of the vagina and increase the risk of infection. Many women say shaving or waxing makes them "feel cleaner," but there is no reliable evidence to show it actually makes one cleaner, reduces infection or odour.
It's depressing though that products and services are marketed at women pretty well on the basis that their natural appearance and odour are ugly, unsexy and dirty so here - pay for this and you'll be acceptable as a woman again.