Arduino has strong positions in hobby market. Their strategy is really simple – everything works out of a box. Speaking of development boards – they are open source with unified pin layout that allow plugging multiple extension (shield) boards. Development software is also open, works on major platforms including Windows, Linux and MacOSX. But probably the winning factor is that arduino is rich in libraries allowing easy code writing without knowing whats going on inside chip.
Original Arduino boards are based on Atmel AVR 8-bit microcontrollers that are limited in processing power and even more in Flash and SRAM . Additionally Arduino libraries are written to be easy usable with sacrifice of performance. If you tried to develop more time critical apps then you know that Arduino’s don’t shine in this position. You can work around by writing your own libraries or simply go non Arduino way – writing your own C code. But why loose ability to use libraries and rich community resoures only because hardware can’t ensure desired performance.