I just want to understand, for example: why is it that Rockbox is designed only for some specific players and not for any (since Rockbox has its own firmware it could work with any hardware that is based on a same given architecture, right) ?
Porting rockbox to a new device is essentially the process of writing the firmware needed to actually run on that device. So yes, provided someone writes the code to do so, rockbox can in principle be ported to any device with sufficient memory, processing power, a means of running 3rd party code, a compiler, etc.
By asking this I mean is Rockbox developed slightly different depending on the specific model of a player (eg: Rockbox is developed differently for 'Sansa Clip +' as it is for the 'Cowon' players) ?
Since the hardware between those devices has very little common, when you compile rockbox for each of those two devices you will be compiling quite a lot of different code for each, at least in the firmware part of rockbox. The application stuff doesn't care too much, aside from things like touch screen handling and color rendering for the D2 (obviously both aren't needed on the Clip+).
Why is it not supported for other good MP3 players like the Philips GoGear Vibe?
Because no one has ported it to that player.