Discord ate the market because it is free and good enough voice quality. Before that everyone was paying for voice server hosting or doing it themselves. Is that smart marketing or just the standard operating procedure for startups around that time?
The ability to easily create servers, invite users to your server, and then make that server your homebase with its own channels and emojis, is pretty novel and perfectly fit into the gaming community which is basically a loosely connected graph of friend groups.
Ventrillo and Teamspeak and Mumble were all good. But you had to assign someone in your friend-group to manage the server. This meant paying for hosting to do it "the right way", and in turn one friend either paid the hosting themselves or you had to figure out how to split the cost. Then if someone else joined the group you had to split it with them, etc.. Some people would self-host teamspeak or ventrillo at their houses so you could avoid those costs, but now you are reliant on an unreliable system of one friend hosting your voiceserver on their desktop computer. This means that router mishaps could send it offline, them turning off their computer could send it offline, or if the teamspeak/vent daemon wasn't running then your whole server is offline.
Skype solved a lot of those problems because it was always online, no one had to manage a server, and it was free. It sucked in just about every other way as a game chat option, but the benefits of no-server-management, always-available, and no-cost, made an objectively inferior product dominate the world of game chat.
Discord simply took the features of teamspeak/mumble/ventrillo and combined it with the service benefits that skype offered. No more server cost sharing and no more server administration. But you still got the benefits of actual game chat servers like voice lobbies (as opposed to initiating calls like skype).
I really don't think Marketing is what made Discord successful. This is truly an example of someone who solved a need. We needed a product like teamspeak/ventrillo/mumble combined with a service like skype. Discord was that creation. It truly solved a problem for gamers. Gamers were not looking to cling to skype, but they were all using it. Discord created a product that fit into the market perfectly and the masses ran to it because the need was so big, and Discord solved the problem that gamers needed. The ease of setup also helped. Sending a single share link that someone simply clicked was all it took to join a server and start talking. I think that ease of setup is also an incredibly under-rated strength of Discord. In fact I would venture to guess that most gamers joined their first Discord server by clicking a discord share link that was sent to them via Skype.