What type of bike are you looking at? go back 30 years, and there weren't too many specialized (not the brand) bikes, now you have a specialist bike type for just about every application you can think of.
Since the 80's you have had a proliferation of additional gears at the back, back in the 80's you had 5 gears on the sprocket/freewheel, now you have upto 12 on the cassette so you are just moving the range from one end to the other.
Add to that, a new brand have some on the scene (SRAM) who have never offered triples in non-MTB applications, and need to differentiate their products from Shimano, who are still on 2x cranks for non mtb applications.
If you want a triple, Shimano offer this still upto Tiagra level on road, and at all levels off road, so there is still plenty of choice (for full bikes, manufactures spec what sells i.e the mass market, not necessary what you specifically want)