Because the absract syntax for a graph does not map directly to a concrete syntax the way lists do. With lists you can just always use the literal equivalent for sublists, but for graphs you need to be able to express true references to otherwise-anonymous nodes. This is hard.

The W3C basically salted the earth for graph languages by promoting their awful XML syntax for RDF for so long -- it doesn't just encourage you to just write things as kookily-flavored standard XML trees -- it's also incapable of expressing true non-tree graphs!

N3 is a much better syntax, but even it doesn't quite get quite to the level of homoiconicty you desire: http://www.w3.org/DesignIssues/Notation3