I mean, she just doesn't have enough skins!!! it seems like champs like 
get all the skins.... I'm really glad riot is listenting to the community and creating skins for champs that don't have a lot instead of going for the "we only care about money" route!!!!! ♥♥♥♥♥♥♥♥♥♥♥
If you take Sona's Ultimate skin out of the equation, she haven't had a decent skin in a long time. And by that, I mean... not many people can afford a 30 dollar skin.
They don't just make whatever has the least or gone the longest without one (obviously), they make what fits the theme they are doing and when they have a great idea for it. Diana is getting a skin soon, Yorick is up for an update so they'd just have to completely redo any skins they make, and I think Urgot is the same. I'd love more for Nami, but I don't see the point in whining about it.
Not that it matters. Myself and numerous other players have stopped buying Sona skins ever since patch 4.13 made her unplayable. I can't begin to count the number of people I've talked to that said they would have bought DJ if not for 4.13. The number of people that gave her a try to see if it was worthwhile, just to quote the exact words, "This champ is shit." or, "This champ does no damage." The number of people that didn't bother testing her out before, playing only 2 games and never playing her again, many of which immediately refunded it.
Just like patch 4.13 has severely injured the sales of DJ and her existing skins, it'll continue to damage sales of new skins as well.
While i do like Sona, and a HeartSeeker skin for her is pretty cool. There are better champs who need a new skin that could have worked with it. Like Taric. Despite having a pink skin already, this could have been much more detailed and actually have a skin that changes his skills!
can't get a skin until his rework, so not the best example...
