I kind of stated my view on this (i.e. reason is lack of documentation and tools) but honestly, as a kid who was amazed by the things he saw on an Amiga in the 90s I would more than happily code for it as an enthusiast in 2015.. If only I knew what tools to use and where to go for the knowledgebase.
When you start coding for a platform, one of the first things you do is imitate what has already been done.. The C64 is easy like that. Language, tools, examples - it's all out there. Amiga.. Not really; it's almost as if everyone who once coded something impressive for it would rather be smug about it than go online and post the code and explain.