I think the main reason is that it had a headstart. I believe that gave it a major advantage on top of the pokedex description. But in general there's a few things that people find attractive in Pokémon design.
If we look at a wider picture we can see some of that in the data we have avaliable. Here for example is three different polls, the 4chan post is the smash percent ranked by number and the others is
rule34.xxx and Tiermaker. The numbers on the right represent NSFW growth in the span of four months so you can divide that by 4 to get a rough NSFW per month number.
If we look at data there's Pokémon that are consistent in high placements AND also in getting a good number of NSFW art. Gardevoir, Lopunny, the eeveelutions with emphasis on Umbreon, Vaporeon and Sylveon, Braixen and to a lesser extend Delphox, and Zoroark.
Most of these are feminine designs and/or had a feminine portrayal like Zoroark in it's movie, which likely helped it a lot in this context, most are also cute. So cute and feminine designs seem to be what people like in Pokémon character design. Of course other factors like portrayal and popularity help, as in the case of Gardevoir and Lucario and to a lesser extent Zoroark, in the main series and also being seen as a legacy character like Lopunny. I also think the eeveelutions show that bodyshape isn't as big of a factor due to how ubiquitous they are in this context and even more so when you factor in how they're becoming more integrated into the mainstream perception of pokephilia.
