Humm, maybe because they are so prevelant in the hobby, we feel they are expendable.
I think this is very possible - they are relatively forgiving species and frankly they are a good looking roach to keep IMO. It's also a big plus that they are sexually dimorphic when they reach maturity and the nymphs are quite cute to the average person compared to other species. I see more B. dubia at shows than just about anything else... they really are a decent feeder and with word of mouth by herp and invert keepers alike going around about them I think their popularity just skyrocketed over the past few years.
Relatively productive, accesible, non-climbing, non-flying, and easy to feed with a decent meat to shell ratio = good feeder roach = )
Not to mention they aren't going to infest your home - not that its overly likely some other species will, but I'm sure the thought of a productive roach that can reaily breed at room temperature like the lobsters and lats draws some popularity away from them still.
I mean, even if you wanted to get another feeder species: you have to LOOK sometimes. B. dubia are everywhere on classifieds, forums, shows, etc. With the exception of a few enthusiasts, there really arent a lot of pure Eublaberus and even Blaberus species readily available to anyone at a reasonable cost to start a feeder colony up. I've been the reptile biz for over a decade, and I can honestly say that even I was ignorant enough to believe that there weren't so many good feeder species of roach. It wasn't until I saw one of Zephyr's ads on fauna that I realized the diversity and beauty of some these guys - now I'm hooked! = )
I'm sure with time more and more people will come around to some of the other species - I purcahsed my orange head colony from Casey and he said I was the first to have inquired about them after the ad had sat for nearly a month, yet he sold thousands of dubia and lats in that time. Most people are just blatantly unaware that so many species exist and can be made good feeders.