My rule for which Pokémon is made sexier in HMGBAPS is "strictly already Human-Like" egg group (with some leeway for the semi human like), which eliminates a very large part of the Pokédex. Then, it was Summer, I wanted to draw sexy Pokémon, practice some furry style, and also had enough projects on my plate that I didn't want to die handling MORE work. Then I was reading Modern Mogal and thought, "hey, I like that style, format's fun, I could try something similar and see where it goes..."
Then for settings, since it was Summer, a resort was a given, and it naturally evolved to "what if it was a Pokémon-owned resort and all staff were sexy Pokémon" and the rest is history. :p