Probably because it's a complete mess. I joined as I was curious and don't really care if I have to delete my Instagram account to get rid of my Threads account.
You follow people and then barely see their updates.
From what I've heard about the feel it's a giant data (=cash) grab. Sorta like how Google+ was a me-too and never took off.
The massive inrush is like what happens when the new restaurant opens in town. Everyone goes once or twice to check it out but if it sucks it will still fail even if it is super busy the first week.