I think some people are just more familiar with 26" wheels because they come from a mountain biking background. Others would like the option of riding off-road with fat tires.

I come from a road-biking background and no longer own a mountain bike. To me, having a touring bike with 26" wheels would just be a hassle with no real advantages. I couldn't swap wheels and other parts. The wheels would be heavier with fewer options from light weight, folding tires. My touring bike has very strong wheels (36 H Dyads with Ultegra hubs) and I have lots of options for 700x32 tires that are relatively light weight, durable, folding and low rolling resistance. I have no desire to tour off road or in a 3rd World country.