It's a sad thing. I remember the activity around three years ago when I was around for it. Though at the same time, I guess it's also a matter for the clans to keep up the activity.
I do wonder, though, what demographic change leads to such a decrease in activity when, apparently, the amount of players overall is rising.
Many attribute it to being the increasing demographic towards kids but I've always disagreed with that. Back in the day most clan players were 13-17 years old, maybe not kids but it was a teenager's section in many ways. I'd say that this sort of age range seriously playing Minecraft is decreasing as Minecraft becomes more of a 'kid's game'. The major change I believe is that back in 2011 we didn't have these massive hub servers - if you wanted to meet people or play in a group a clan was a really solid way to go! People knew about them because these forums were the centre of the Minecraft community, everything would be done here and clans were under 'general' meaning that they were one of the first things you'd see when browsing the forums. Massive attention, massive interest at a time where clans were thriving! But think of how difficult it is to find this section now, and when one arrives it's a section full of dead posts or tiny startups looking for a factions group or YouTuber group - the lack of clans, in general, has led to a massive spiral of decline.