27.5" is 650B (as I recall) and has been around for a long time... just not common. I suspect the flooding of the market is just a marketing ploy (or possibly a fad, which does often drive cycling products). After all, if 26" is good for some things, and 29" is good for others, then 27.5" must be the best of both worlds.

I suspect there was a slowdown in sales of mountain bikes with both common tire sizes, and so the "new" size was introduced to boost sales.