Been awhile i been here.
Anyhow main reason why Misty parents weren't showed is because they weren't relevant to Misty story or plot at that point.
For same reason Ash and Dawn father, Iris or Cilan parents weren't brought up either.
This has nothing to do with how important traveling companion in this show is, but in whether writers find diving more into someone past and history is required to resolve issues like it was case with Pewter gym and Brock dis functional family, or be done for sake of more substantial story with purpose of learning more about character past giving it depth in aim of achieving significant development with character having him grow further emotionally.
That said i always thought some info about Misty parents would have been intriguing to see and set up foundation for quality driven story twist. Considering how family tradition in Cerulean is to mainly use water type, her parents could be some recognized water specialists which left on journey to achieve mastery over said type . Which could provide some good character development in having their daughter not wanting to fail their expectations or ending disappointed with her causing anger and revolt in wanting to prove them wrong. But also shatter her confidence not being sure if she is cut out for such task.
Often someone relatives can become important to storyline later on if writers decide to use them as vectors,being possible to do some great development by just exploring more on someone family or past. Touche it wasn't meant to happen.
She surely was great character and hopefully writers will give her some recognition and screen time in future, whenever that would be. She can certainly offer more and there exist many unanswered things about Misty story which would be interesting to expand upon and give her some more development.
Haruka was much more important character in anime than Kasumi. That's why her family was introduced (beside mentioned reasons).
Kasumi was just side kick for Satoshi in the first seasons while Haruka was a MC.
And still they featured Kasumi's sisters.
I wouldn't necessarily say she was much more relevant, its more she had bigger role. May was relevant in aspect of introducing contests in this show being first female protagonist. While Misty importance derives from being Ash coach and motivator pushing him to go forward along with Brock . Brought humor, tension and lot of flare in group interactions, often played role of classic heroine giving tom anime identity with her strong presence.
Along with having her motives and desires, quest of becoming water master, background etc paving way for future females to come in anime.
Both left big impact on anime and fandom in their own ways.