Ubuntu on the Nexus 7

In most of the world, you buy the phone separately from your phone plan, and a dual sim is an advantage, not a disadvantage.

How do you measure "most of the world"? By number of users or by amount of money spent on phones?

Most affluent customers live in countries where carriers sell mobile phones tied to the contract - and of course you'll not see dual sim phones sold by carriers any time soon. And makes no sense whatsoever to produce high-end phones for cheap countries like China or India.

There are upmarket dual-sim phones from major brands too, like the Samsung Galaxy S Duos and some HTC Desire models.

Is this some kind of joke?

Samsung Galaxy S Duos (July 2012):
480 x 800 pixels, 4.0 inches
768 MB RAM
4 GB internal memory
1 GHz Cortex-A5 (single core)

HTC Desire VC (June 2012):
480 x 800 pixels, 4.0 inches
512 MB RAM
4 GB storage
1 GHz Cortex-A5 (single core)

HTC Desire SV (November 2012):
480 x 800 pixels, 4.3 inches
768 MB RAM
4 GB storage
1 GHz Cortex-A5 (dual core, finally, but still obsolete architecture)

Compare it with old, obsolete, and no loner top-of-the-line phone of 2011:

Samsung I9100 Galaxy S II (February 2011):
480 x 800 pixels, 4.3 inches
1 GB RAM
16GB/32GB storage
1.2 GHz Cortex-A9 (dual core)

All these upmarket dual-sim phones are still less powerful then by now almost two years old phone! Heck, they are still stuck with WVGA when top-of-the-line phones had WXGA for more then year:

Samsung Galaxy Nexus (October 2011):
720 x 1280 pixels, 4.65 inches
1 GB RAM
16 GB storage
1.2 GHz Cortex-A9 (dual-core)

In short, dual sim has nothing to do with anything.

If "dual sim has nothing to do with anything" then why all dual-sim phones have two years old specs?

It is true that only low-cost devices seem to have qwerty keypads these days. I attribute it to brainwashing by Jobs, and/or cost-consciousness on the part of the relatively discerning customers who use the phone primarily as a communication device and demand a qwerty keypad.

WTF?

Motorola DROID 4 (January 2012)
540 x 960 pixels, 4.0 inches
1 GB RAM
16 GB
1.2 GHz Cortex-A9 (dual-core)

It's resolution is fine for it's size even in 2012 and it's still more powerful then all the upmarket dual-sim phones

I'm not sure why people are not buying high-end QWERTY devices, but that's not because there are no offers: there are still some decent QWERTY phones but they are ever less popular. There will probably no more offers in the 2013, but that's because people are not buying them.