I don't think anything has really moved to Discord as much as people have just tended to be more active there naturally.
Personally I'd say it's, unfortunately, not too surprising as forums in general really have been quite falling out of fashion in the last 3-5 years in favor of other social platforms such as social media websites/apps such as Twitter, Reddit, Amino, Tiktok, etc; as well as instant messaging services such as Discord, Telegram and Whatsapp.
I agree it's a shame to see the forum having dropped in activity in recent years, especially as one who has been around to see it thriving in much of its prime, but I feel this might just be the natural progression of how the internet culture has evolved.