It got forgotten during the period it was on Brock's team. He gets it in season six and it evolves to its middle stage in season nine, by which point Ash and May's Hoenn starters had spent over a season and a half in their middles stages (and would evolve to their final stage in the second half of season nine) as well as getting regular use in gym battles and contests.Gen 3 is going to be 20 years old in November... NOOOOO
Also I’m convinced the writers have completely forgotten about Brock’s Marshtomp which depresses me