I have no idea why Arduino is so popular--it has NO debugging support unless you use an ICE. (People "debug" with printf statements.) We use Netduinos because with .NET we can single step and inspect variables in live code.
Why is Arduino so popular? Brand name recognition, a decent standard library, and a ton of sample code and tutorials online.
I agree that it's not a great platform, though. I personally prefer STM32 these days; it's dirt cheap (a basic development board is <$5) and powerful (72 MHz / 20 KB RAM / 128 KB flash on one of those boards). I believe there's even a way to integrate it into the Arduino IDE nowadays!
Arduino is popular because it was the first one that got big and built up a community. It's the same reason the RPi is so popular despite not being especially compelling from a pure spec standpoint.
You can get more performance for less money with other boards, but you'll have to figure out a lot more stuff on your own.
IME Arduino is so popular because it's so cheap. I can buy Arduino ProMicros on Alibaba cheaper than I can buy the Mega168 MCU itself. And that's not counting the PCB, additional chips or board.
I agree that it's not a great platform, though. I personally prefer STM32 these days; it's dirt cheap (a basic development board is <$5) and powerful (72 MHz / 20 KB RAM / 128 KB flash on one of those boards). I believe there's even a way to integrate it into the Arduino IDE nowadays!