Matrix was selected from a large number of chat providers. Here are the most important features in brief: 

Federation:          
Matrix enables federation, which means that different servers can communicate with each other to exchange messages and other information. This allows users on different servers to chat with one other without needing to have an account on the same server. As a result, it is much easier to communicate with other scientific institutions that use Matrix.

Open-source software:
Matrix is an open standard that is developed and maintained by the Matrix.org Foundation. This means that developers around the world can develop Matrix clients and servers that respect the standard and thus contribute to interoperability.
In contrast to centralized messaging platforms such as WhatsApp or Facebook Messenger, Matrix is decentralized. This means that there is no single central server instance that controls all data traffic. Instead, the University of Freiburg operates its own server through the University IT Services.

Encryption:      
Matrix supports end-to-end encryption to ensure the security and confidentiality of messages. Only the participants in a conversation can decrypt the messages, i.e. even the server through which the messages are routed has no access to their content.

The following features also help Matrix Messenger stand out:

Detailed information: https://matrix.org/faq/#intro