The Raspberry Pi is conceptually quite different to the Arduino platform, it is a "single-board computer", compared to a microcontroller board, which is an embedded device platform. Different purposes and approaches. Typical uses of the Pi are in areas one would not consider an Arduino for, and vice versa.
The other microcontroller platforms mentioned too have little in common with the Arduino "universe", in that the 'duino world is all about a particular set of open hardware board reference designs (clones and all), with a particular set of bootloaders, supporting a couple of Arduino-defined standards of "shields", and programmed through a particular IDE, the Arduino IDE.
Sure, the same Arduino boards could be used without the bootloader, and without the IDE, but that would then take away the advantage that has provided the Arduino with so much traction: A common set of code norms and functions, a vast community of practitioners, and a mission statement that specifically precludes both EEs and programmers: "It's intended for artists, designers, hobbyists, and anyone interested in creating interactive objects or environments" (from the Arduino official site).
The other MCU boards out there may have similar advantages, but if they had such a similar strong support system for the practitioner rather than the technical user, they too would have had the ubiquity and wide user-base that the Arduino enjoys today. If any such board gets to that point, I am sure there will be enough proponents, and enough reason, to propose such a site.
That time is not now.