Because XMPP always took ages to implement basic features such as server history, encryption, file upload, group chats etc. and even then they had multiple comp

Because XMPP always took ages to implement basic features such as server history, encryption, file upload, group chats etc. and even then they had multiple competing standards. This had only recently been solved by few individuals, however the problems with this is that those individuals are very opinionated. Not a bad thing in itself, but ensures that certain other features will never be implemented. Matrix in contrast control the reference server and the reference client, and have enough resources and internal diversity to implement a diverse featureset. And, XMPP never tried to solve efficient multicast problem.