Your question presupposes that StackExchange and RPi specifically should provide an answer to everyone's question.
One of the strengths of this site has been its narrow focus. There are an ever growing number of boards including, orange, banana etc. While some are similar and even run Raspbian etc., there are also some significant differences, which this community may not be equipped to answer (lacking experience with or access to a physical board). So raise the same issues as the Arduino vs. Pi question @goldilocks mentioned.
One of the strengths this site has had is its narrow focus (Raspberry Pi only). It has allowed us to grow a community of knowledgeable users willing to help others. To provide a little context the Pi was the first of the Pi boards (orange, banan etc.), and has sold (several million boards) a number I doubt any of the others can even approach.
As @goldilocks pointed out this question has been raised repeatedly. You can gain some insight into the thinking behind this issue by exploring the links on the right.
A new SO site proposal may fail for many reasons; but the relative sales figures for these other boards, may hint that lack of a demonstrated need or too small a pool of knowledgeable users to service that need. Again, can or should this site or SO as a whole be all things to all people or will that only serve to diminish the site and brand.