Why does it have to be a mailing list? Can a software project communicate with an interface platform more like HN/Reddit? For that matter, could you use a subreddit for the purpose without obviously violating Reddit's TOS?
Some do. It depends on the nature of the development. Mailing lists are a very popular format. One of the other projects I manage communicates near-exclusively through IRC. But good mailing lists have the main feature of topic-centered discussion, usable with just an email address (very low barrier of entry, easy to add new people to the conversation, easy to continue a topic in private).
Voting isn't generally a feature you want for discussions - voting provides visibility over a short period of time, and then the topic dies off, which is a very big issue with reddit-likes being used for discussion. Newcomers to a highly popular topic are on equal footing with the rest of the participants, while on Reddit/HN the topic is overwhelmed and only the highly popular, old comments get visibility.
This is very suitable if you don't want everyone to have equal footing. For example, discussions centered around video games, politics, social issues, etc. For open source it tends to be bad. This is an off-topic meta-discussion I'd love to take further, in private, if only it'd take me a click to do so. :)
