"In a homoiconic language the primary representation of programs is also a data structure in a primitive type of the language itself. This makes metaprogramming easier..."

For lisp, that means programs are are also lists, so can be manipulated with macros easily (because lisp macros are also lisp, which is good at working with lists).

For a relational language, that would mean the programs are also relations, which would be a little weird because relations are unordered.