It's a simple protocol that you can easily write software for.
It is usually not hosted by a corporation that has its shareholders to answer to.
It is an open protocol.
Since it is an open protocol, you can use whatever client you want, which means you don't have to deal with pop-up ads and begging for money whenever you sign in, and can use it from a terminal instead of needing a GUI app that is probably written in JavaScript and forces new, unwanted features and UIs on you.
It is once again obscure enough that the Eternal September is abated a little bit.
Software development has long been discussed over IRC, so it is part of a certain subculture of folks who are involved.