Once it was obvious that emailing was a thing many people wanted to do, and a free server became available, servers started springing up all over the place, many more implementations of the underlying protocols were developed, businesses figured out various ways to monetize, the standards evolved (somewhat).
Now let's do an inventory on ActivityPub...
1) It became obvious that tweeting was a thing many people wanted to do? Check
2) A free server became available? Check
3) Servers started springing up all over the place? Check https://www.similarweb.com/blog/wp-content/uploads/2022/12/m...
4) More implementations were developed? That's where we are now
5, 6) Stay tuned