Himeaimichu

Grieva often sing about murder and suicide, Gossip have a lot of topics about nationalism/anti nationalism etc.  But yes, Kuroyuri have similar topics so they should have that parental advisory as well I guess

It could be that Grieva and GossiP are more blunt with their lyrics. I mean, Mei tends to put LOTS of hidden metaphors in his lyrics and such, though I'm sure Kyouki and Saku do too. 

Or it could be the way Mei sings, since he sings in a more archaic way, using weird Kanji readings, and occasionally using obsolete phonemes such as "ye", "we" and "wi". But then again, so does Kyouki. Saku seems to sing in a slightly more modern way compared to Kyouki and Mei, but still somewhat Archaic. I have heard from a lot of people who try to translate Kuroyuri songs that Kuroyuri's lyrics tend to be really hard to understand due to Mei's grammar, because Mei is known for using the fluidity of Japanese grammar. Also, it's worth mentioning Mei slurs his vowels in different ways a LOT.  I've also heard Kyouki's lyrics are a little harder to understand, but I don't have much info on Saku's lyrics, since not a lot of GossiP's songs have translations. 

(If only I actually spoke Japanese xD. I could tell you all about the language theory and the archaic forms of the language, but I can't even speak it xD)

But the most likely reason is probably something so simple as "Kuroyuri doesn't use any English cursing" xD. We'll never know, probably. 

Update: I just realised only 2 GossiP releases have parental advisory, and only 2 of Grieva's. IDK what this means, but it's probably useful somehow in finding out this honestly arbitrary thing I just thought xD