[atoav]

That is what I meant: the 555 is a habitual option by people who grew up with it. The arduino can be an habitual option by people who grew up with that.

Not every project is mass produced or must be highly optimized when it comes to size, cost or power consumption. People use what they know, and what they know depends on when they grew into it.

[6SixTy]

Arduino is very much less about the board itself, and more of a software framework. Every single "Arduino compatible" board beyond the standard Uno or Micro have a bunch of macros essentially just telling the compiler what the standard Arduino pin definitions go where. That even extends to boards like the 2560, since I know the port registers and the silk screening on that board are completely different.