I'm not going to lie and say that it doesn't that they're not in it that much, but it's only been a bit over a year. They're still getting a grasp at line distribution correctly, especially M, because you have the most singers in that group, while K has more of the rappers, so of course line distribution is a lot harder to do in that group. Plus, it just depends on what songs they do, some songs in XOXO don't even have Kris singing/rapping, while he's just in the chorus. It all depends on what type of songs they are doing and what voices are needed. And in M, Luhan and Chen just have wide ranges that fit a lot of songs, but in K, Kyungsoo and Baekhyun get a ton of line distribution because they're the main singers of that group.

It stinks, but rust me, they actually have better line distribution than most groups. Take Infinite for example, it's been 3 years and Sungyeol still gets one line per song. Yeah, he's not the best singer in the group, but to only get one line per song is pretty terrible, especially after 3 years. But then again, Sungyeol became an idol to go into acting instead of singing, but it still because it's not like he has a terrible voice, it's just that Woohyun and Sunggyu are just really good singers and Myungsoo and Sungjong have improved so much since their debut days that it's hard to give Sungyeol more lines some times, while Hoya and Dongwoo take care of rapping and dancing for the most part.

However, what most people don't realize is that in concerts, the members get to do their own stages, so Xiumin and Sehun will get their time to shine a bit. Plus, depending on what shows and talents they have, they could easily fill in MC spots, do acting, or even varieties. It all depends on what the band needs and how SM is managing them. So Xiumin or Sehun may not get to sing as much as they want on the albums now, but it won't be like that forever, especially if SM ever decides to do more subgroups like they had with SJ.