-- the universe won't wait for you
Hmm. Well, I would say that Misty's parents were never really relevant. She and her sisters seemed pretty capable of living without parents and they had their gym and performance and things. Maybe they were dead, maybe they were travelling, who knows? But they weren't really relevant to her character. Brock's family was more relevant, being that he was taking care of a ton of siblings after his mom and dad left. It was only by his dad coming back that he could leave the gym and travel. That was important to the story and his character. For May there was a good reason to feature her parents as well- her dad is a gym leader. Her brother travelled with the group. The family dynamics were a prominent theme in multiple episodes. Even if they had gone the other route and had May be Professor Birch's daughter, he still would have been featured. There is really no escape from meeting May's parents unless there was a drastic departure from the game.
And I object to you saying that "all May did was contests". Contests may have been a small part of the games, but in the anime they were hugely popular and widespread, seemingly even more entertaining for the common person than gym battles. And that girl accomplished a hell of a lot. Misty, sad to say, didn't get nearly as much character development as May.