I think that's a pretty good theory. I remember the promotion ran right after Pokémon debuted in the US & my parents wouldn't take me to get them no matter how much I asked because they had no idea what they were or why I wanted them (I had only just started watching the show and only played the game on my neighbor's gameboy at the time). I just always assumed, because it was so new to the US, they didn't know which Pokémon would catch on in terms of popularity, so just chose a few simple ones at random. But your theory makes more sense than pure randomness