I'm not entirely sure why that is but it seems clear forums are having a hard time attracting younger members.
It's all down to attention: which platform provides the most (perceived) attention for members. That's what drove forums in their heyday. That's what's driving social media now.
The psychological term "spotlight effect" describes well the motivation that many derive from social media activities (and formerly from forum activities):
And then there's this famous cartoon, which beautifully underscores the futility of it all: