payload maps are way too big and offclassing/full-timing sniper on them is really boring (go ask any highlander player how they feel about sniper at all times)
a/d is lowkey kind of fun in theory but the maps available for it are all kind of goofy or bad, like kawa mentioned gpit used to be part of the official comp rotation but it fell out of popularity.
ultimately the game is so small and insular that questions like "why are there only these classes, why are there only these gamemodes, etc" can be answered with "we have tried a bunch of stuff for 16 years and we've decided these classes and these maps are the most fun"
most people i think that are new to competitive tf2 are often unsatisfied with hearing that because they haven't had the time and experience to find out why things like full time pyro and sniper are boring and not fun for either team, or why 5CP is one of the few functioning modes for a team size of 6. they're also just generally unsatisfied with hearing that they cant main their favorite class in 6s which is an understandable complaint to have but it'd be like being mad that your favorite gun in csgo is the auto shotty and you can't just use that every round and do well at high level CS