This, tbh.
Well they sell more albums than most (they outsold everyone but EXO last year), members are on tv every day, or at least almost every day, via MC gigs/variety shows, many members have acting careers (Siwon was just in People magazine for a movie, Kangin did a movie, Heechul and Donghae have acted too), Sungmin, Kyu, Wook and Yesung have done musicals successfully, they get a good amount of endorsements, and SuJu is one of the only K-Pop acts to have SOLD OUT WORLD tours on a STADIUM level. Plus Kyuhyun had a huge solo hit in 2014, which will likely chart again this year - and yes, individual recognition counts since we saw that via Gaon's ranking with Big Bang last year. Plus their MVs get top-tier level views (Mamacita was the third highest viewed MV by a male group after Overdose and Good Boy (which we all know YG had some PPC going on...).
On the other hand their songs don't chart well in Korea - that's their only problem of why they 'aren't top tier' - but it's not like DBSK's songs do much better...Globally SuJu is one of the most popular K-Pop groups. Even if their music isn't charting well, people in Korea know who SuJu are...
Yep, they're Hallyu Kings, but they're not as big in Korea. You should know what Hallyu means since Hallyu is about popularity outside of Korea/Korean exports to other countries...