Perhaps they feel that the risk of this to their brand identity is not worth it. And a third-party app would not give Nintendo the same level of control that it normally has. We all know that there are some pretty toxic things in the gaming world. There are also questions about the impact of a chat service like Discord, and the family-friendly approach that Nintendo tries to take. This means they tend to be slow at giving their development tools to third parties. As we know, Nintendo tends to be very protective of its intellectual property.
And there are probably a host of reasons for this. In the years since, little appears to have changed to make that happen.
