Q:

Why is the single-rear derailleur that enables <= 12 speeds so popular?

All the high-end MTB, and road racers seem to have somewhere between ... 6 and 12 gears. NO derailleur up-front, only a few to scroll through in back. I get the KISS principle behind it; I like having first gear made extra-extra large to make up for missing lower gear elsewhere. Seriously why is this so popular? All the $5000+ bikes have ONE deralleur and limited gears. Is it really better to design gearsets this way? Why? Is it faster? More durable? 

A:

Any less than 21 speeds for that kind of jack seems wasteful to me.

You must understand that what matters is not how much "speeds" drivetrain offers, but the available range and other factors (see below). Let's compare 3x7 13-30 24-32-42 and 1x12 10-50 36: 3x7 has 404% range versus 500% range of 1x12. That's a 96% difference! 1x12 will also be lighter by at least several hundred grams. The quality of parts will also be generally higher, given that there are no modern high end 3x7 complete groups. Even compared to a 2x10 11-36 28-38 444% 1x12 has a wider range.

Seriously why is this so popular?

Why people choose one-by:

Similar or wider range with reduced simplicity.
Weight reduction.
Frees up bar space for remote control levers (suspension lockout, adjustable seatpost).
Availability. SRAM does not offer 2x12 or 3x12.

And why not:

Cost (in some cases).
Maybe a reduced service life. A lot of factors contribute to this, I think it's a complex matter.
Limited availability. Good luck finding a replacement 12 speed chain in third-world nowhere.

Is this a fad? Is it really better to design gearsets this way? Why? Cuz Shimano said? Is it faster? More durable?

It's a legitimate trend that offers benefits for some people at cost that other people might not find acceptable. A general advice would be to use what you find suitable for your application scenarios. Manufacturers might have their own reasons to reduce front gears amount, hype/trends and manufacturing costs being among them. Only companies are able to answer about particular reasons, but I doubt you'd hear anything beyond approved PR talk. Not like it has any different for a bike industry since always.

