>>25301359Swampert got popular by itself first and foremost, which IMHO speaks about the design quality more than any marketing push ever will. It's base form is extremely cute which initially drew players towards it and the final form is quite badass which cemented it's popularity among the fans. Unlike the other two it's design is also pretty simple which made it quite appealing to people who appreciate the simpler designs of gen 1 and 2.
The main thing that bolstered it's popularity later in life was probably the Mudkip meme which made every normie mememaster have a Swampert nicknamed MUDKIPZ at least once in their lifetimes. It was also included in Pokemon Special manga as the starter Pokemon of the main character Ruby, and RS saga is considered by most PokeSpe fans to be the best in the entire series. It was also competitively the strongest of the entire trio during gens 3 and 4, and arguably still is if we assume Battle Spot Doubles is the main meta GF balances their game for.
The anime writers IMO made a mistake by choosing Sceptile as Ash's ace Pokemon in gen 3. They probably assumed that a lizard / draconic Pokemon would be the most popular choice among players due to Charizard's retard levels of popularity, but they didn't consider that Grass Pokemon are just flat out not popular among the players period (especially after the colossal failure that was Chikorita in GSC). It didn't help that Sceptile was inarguably the worst starter pick in RSE, struggling against practically everything beyond the first Gym. The levels of their failure still shows in various popularity polls where - despite seemingly having so much push - the Treecko line still ranks as the least popular of all Hoenn starters.
