I don't think it's the concept but Liz's skills as a vocalist aren't really valued in IVE. All of the members have really distinctive and I guess loud voices. Wonyoung and Leeseo are high and nasally, Rei is also high but with a little spunk, Gaeul intonation is really unique, and Yujin is deeper and powerful. Liz has a lighter and beautiful vocal tone. It doesn't stand out to me as much and could be why she isn't getting as Main Vocal type lines and high notes. I think her vocals would be higher valued in a group with a more mellow concept and less 'loud' vocal tones.
I hope this makes sense and I'm not trying to insult any IVE members. That's just how their voices are in my head.
Miyeon doesn't get to show off her vocals even though she's the best vocalist in gidle. I think this is also because her tone isn't as unique as the rest of the members despite it being unique in it's own right. The group is also not super vocally heavy. So she gets the least amount of lines a lot now and sings similar parts.
I think in Kep1er a lot of the members don't shine in terms of dancing because so many of them are great dancers.
Usually Dayeon, Hikaru, and sometimes Xiaoting are pushed for dance breaks. But members like Yujin, Youngeun, and Yeseo are phenomenal dancers but aren't treated as such.
Seulgi is the best dancer in Red Velvet and typically is treated like Main Dancer. But her dancing style doesn't suit Red Velvet songs, especially the cuter ones. She's better at hip hop oriented dances and open style. She really shines in videos of her dancing to non Red Velvet songs.
Lee Know is the best dancer in Stray Kids and is kinda treated like a Main Dancer. But it's really Hyunjin who gets the Main Dancer treatment. I think this is because Lee Know has a more controlled and fluid style of dance which typically doesn't fit SKZ's concept. Hyunjin dances powerfully and stylishly. Even Felix is favored at times. But I also think this is because Lee Know was a back up dancer and hasn't been able to ditch the muted style of dance he adapted.