I suppose a company can not be PURELY open source software & hardware, otherwise making a profit would be very difficult, especially with the globalized economy. The Chinese will always make it faster, cheaper, and although not the best, good enough.
Plenty of bootstrapped microcontroller modules are out in the market. Why did Arduino catch on so immensely? My guess: low cost, the choice of a common language (C), floating point math, analog-to-digital converters (missing in basic stamp) & other peripherals, a very good forum, and "open source". But the same can be said about leaflabs.com, so what is it about Arduino? How did they make it into Radio Shack stores?
Add to the attributes of the initial arduino platform success is the fact that they released the IDE in three major OS versions, Win, Lin, Mac. Not sure any other offering at the time had that avalible as standard. As far as Radio Shack goes, they are a pretty late to the show as far as distribution goes, but can only help add to the user population. Only time will tell if RS continues to sell them as I'm sure they will drop them in time if sales don't meet some minimum expectations. At this point in time I think RS needs arduino more then arduino needs RS, as RS is having real issues with trying to figure out what their core business should be these days. They do seem interested at trying to at least explore going back to their early roots rather then just being a cell phone and Christmas toy store.
Mix is a little luck at having the right product at the right time at the right price with the right 'features', helped put the arduino platform on a successful track. I'm sure if you asked the project originators they would admit never dreaming it would be such a popular platform and that their original goal never involved trying to be the #1. There is certainly nothing technically superior about the arduino, either in hardware or software, compared to other offerings either then or now. I personally was attracted to it because it seemed to make learning and using C/C++ a lot less daunting a task compared to anything else I had come across at the time. Most 'beginner' platforms used some proprietary form of the Basic language which is always somewhat limiting as far as growth and portability goes.