Short answer the timing sucked for them pretty much always.
First of all fx debut in 2009 mostly because they had to get Victoria to work.
Most of f(x) were children at debut
Victoria 22
Amber 16
Luna 16
Suili 15
Krystal 14
Which meant that most of the group was curfew locked. So Victoria was hard carrying them. In addition to being the first Mainland Chinese group member to gain real public affection in Korea. So Vic had loads of stress.
Krystal was acting mostly as a younger sister in a sit com.
Suili had done some acting since 2005
because of the style, ages and member choices.
The fx fanbase was dominated by more aggressive solo stans than any group i have seen in kpop.
Music wise they had a digital single debut. LaTacha
followed 3 months later with a physical single Chu
spring of 2010 saw NuABO mini album.
Amber got hurt just after NuABO and was out until the next album in 2011.
So that was a less than idea first 18 months.
SM at the time had The DBSK stuff going on ( ie them going from monsters to court and the weird break up)
2 monster groups in SNSD and Super Junior.
Shinees debut what was just crazy.
So just to get effort from the company when anything SNSD or SJ did was making buckets of cash and both were touring, was hard.
The 2011 release was the first full album and it did well by the standards of gg then and even got a repack. 70k was very big for gg
very very few gg were touching that number. ( for reference SNSD never did less than 100 k)
Pinocchio (danger) got fx their first wins.
2012 saw 1 mini album from f(x)(a monster Electric shock) this should have been the year to focus on them {because SNSD was in Japan and released nothing all year, but the tts sub unit. } but the members were doing solo projects.
2012 was the beginning of the end Somewhere in here focus changed. Suili wanted a different life is my read. This was year 3 every sm group tours by year 3 at the latest. no tour for fx.
2013 saw arguably the biggest album and song from f(x). Pink Tape and Rum pum pum. but behind the scenes the wheels were coming off, other members fans were on Suili like crazy convinced she was dating. To the point that there was strong feeling that she was going to leave the group.
That did no happen then.
2014 full album 3 Red Light another critical success and popular, but the afore mentioned issues with Suili came back with interest and she left the group mid promotion to act only. (2014 was another train wreck year for SM as a whole also)
2015 the remaining 4 came back with 4 walls again a critical and sales success. Krystal now hated the ground snsd walked on. fx had their first solo concert and sm made sure snsd was in Thailand for the dates.
then Victoria went back to China to work there in her homeland and that was when relation with both countries went to hell and Vic posted on the Chinese side.
Musically as a group the public liked fx. The solo by Amber and Luna not so much.
They loved Amber in variety, Luna in plays, Krystal was just an it girl and actress. She and Jessica were a crazy power siblings, and were doing tv shows and the rumor is a mv and album were done. But Jessica getting thrown out of snsd killed that and really pretty much killed fx.
Suili's death is another burden the members just struggle with.
Luna says f(x) is still around but with none of the members in SM. None of the girls fighting for rights to the f(x) name or brand (unlike say gfriend) that seems not to plausible.
To Drew's point about the lack of fx in sm official goods there is not stuff for group like the grace or shinwah ect either.
The girls in both rv and aespa mention them as do other gg. SM has come down on the side of just not fighting the fans who say they would be money grubbing by keeping them in the pantheon. I think there is a way to do it but selling SM on one would take a bunch of work.
edit btw shouldn't this count as like 8 posts
@OnlyCalB jkjk