"I'm far too British for this…"

I would say that the waifumons did not becomes a generational staple until Generation III introduced Gardevoir. After that, we had Lopunny, Gothitelle, Delphox, Tsareena, and now Hatterenne (with honorable mentions being Zoroak and Florges). Gardevoir still remains the #1 waifumon despite all the challengers who sought to dethrone her. I would credit the simple elegance of Gardevoir's design (Gardevoir is the easiest to draw out of all the Pokemon I mentioned).