Why are Atmel AVRs so popular?
The AVR family has a lot of good, inexpensive, hobbyist-friendly devices with nice peripherals, low power consumption, and good cross-platform support.

Yes, Arduino is a big part of it.  But I think that Arduino came to exist the way that it did -- and to the success that it has -- partly due to those features.


Good:  They work well.  Easy to program in C for most basic functions.   Adequate documentation.
Inexpensive:  Lots of $3-$5 parts, available from major distributors in small quantity.
Hobbyist friendly:  Parts in through-hole packages-- a big contrast to many of the chip families out there today.  Newer AVR (e.g., xmega) devices are less so.
Nice peripherals:  Built-in oscillator, flash memory, on-board RAM, serial ports, ADC, EEPROM, and the other goodies that make it possible to run a single MCU on a protoboard to do basic stuff, without too much hassle.  
Low power consumption.  AVR's major pitch point these days.  Suckers can run on a battery almost forever if you know what you're doing.
Good cross-platform support:  The AVR was designed with C support in mind-- not as an afterthought.  GCC support came early, and a big open source community developed around that.  It's still one of the best MCUs that you can develop from any platform with free tools.  This is a big one with respect to the other families, many of which use proprietary compilers or have lackluster gcc support.  Even PIC was pretty late to the game with good free C compilers.


As for why there wasn't much about it in the replies to your earlier question, I think that (1) you're seeing small sample bias and (2) many of the answers were specifically to discuss non-AVR solutions-- because so much of the discussion on this site is AVR/Arduino-centric.   Most of the microcontroller families aren't represented in your list as of this writing-- including some that I use regularly, and others that are among the most popular in the world.