I think that it was more than just marketing. Prior to Arduino, it as hard to get started in working with microcontrollers. Almost every manufacturer focused their products on already trained engineers. Arduino, from the beginning was primarily targeted toward learning for beginners.
1. Arduino was cheap
2. Arduino did not require specialty hardware for programming
3. The IDE was free, cross platform, and worked out of the box without any complicated set up.
4. They focused on lots of accessible documentation and learning material.
Now that Arduino has been successful, everybody else has jumped on the bandwagon and in many ways have developed superior ecosystems. But I credit Arduino for being the trailblazer. I have recently reallly been into MBED and Spark Core, but I doubt that those systems would exist as they are today had it not been for the creators of Arduino.