Please appreciate that we are volunteer contributors - we do not make the call to use a product or service in Mozilla.
Although Discord is used by the Hubs team, for pan-Mozilla asynchronous communication we use Matrix. It’s has good controls to protect the community and the team at the back of it are big supporters of open source.
Part of the criteria used by any organisation in deciding which product or service to use is looking at the need for that product or service - although there may be guidelines, there is not going to be one set of criteria for everything. In respect of the process that led to Matrix, it was done as openly as possible and with feedback from the community.
Personally, as some one who joined Mozilla through IRC, I like Matrix. Given that Mozilla does not have to provide this for the community, we have an open, well managed platform that gives us features such as scroll back that helps us contribute asynchronously.