Forums are becoming stale and obscure. Just like how nobody uses old-style phones anymore, that's how forums will become eventually.
Forums are taken over by other websites like Discord and other chat websites.
Many people here have grown older, leading to them growing out of this place as well as losing interest in SpongeBob. They're probably dealing with other things in life as well.
Going back to forums, Discord, phones, and the show, cable will likely become obscure as well, due to the video streaming sites becoming more common nowadays.