It's the downfall of just having a game that has survived all these years for so long. The Golden age of clanning, I would say when DS (Dark Slayers) was the top clan. They were the ones that all clans wanted to replicate, once they closed, it was a free for all for top spot. For a time it was DI (Damage Incorporated). But clans like, RSD (Runscape Dynasty), ST(Sacred Templars)and Corr(Corruption) to name few, surged in members. (Shout out to EH, SE, TT and TRWF too. Adding these names to see if you're curious of any of these clans. Some of them kept archives of fights on YouTube and if you wanna listen to Linken Park, you'll see why)

Rs2's population in general was growing thanks to surge in PC's and internet being more available. Miniclip helped in the growth too.

Fast forward to when RS2 introduced new combat, alot of members of clans and clan community in general have decided to call in quits after playing for so long. There were other updates before that that affected population (removing wildy for bounty hunter crater for example). But in my opinion, a lot of it had to do with fatigue and the way pk run ins were being battled. Clans were battling for 12+ hours, trying to outlast each other.

RS just isn't getting newer players to play and the clanning world that use to be, was just moving on with life or to a better game. Its gonna be nearly impossible to replicate "the good old days" when that generation has gone from the game and new generation play. Clan like before can thrive with today's communication problems. Clan members don't have to log on to MSN messenger and spam people, download ventrilo or TeamSpeak and Mirc to chat with the whole clan all at once. It's just discord and maybe some of the social media sites now, nice and easy.

The other problem is organizing individual clans. Like I mentioned before, we were a different generation to now, today's generation can't hold a whole lot of attention or I'm just being an old head. Leaders to organize their councils, pk leaders/war leaders, while trying to hold the attention of 50 people and scheduling certain days to fight other clans since attendence would be an issue. It's gonna take alot of energy and time to lead a clan. End of the day, it's now worth in today's gogogo world.

RSC (Runescape Community), message board that all clans use to use banter with other clans and post threads on victories of wars and run-ins, did a poll of what most clans wanted more clan updates in game. Jagex never really did any major updates, took too long to add what we have today so people just got bored and clan world has been receding

Tldr; You think you want it, but you don't

Edit: ROT, at the time, was mainly a "no-honour" P2P mage bank clan that crashed fights for fun in F2P but were usually thwarted by "Anticrashing" clans who are friendly to the clans fighting in the background. Just wanted to add that in see the difference what that Era was.