It is free, and it has always had good push-to-talk voice chat using a permanent "room" concept (and not a "call" concept as is common with business-focused too

It is free, and it has always had good push-to-talk voice chat using a permanent "room" concept (and not a "call" concept as is common with business-focused tools) suitable for gamers, towards which it actively marketed itself. This stuff has been in use by gamers for decades, but previous solutions either depended on paid servers or required one person to set up their own server on their own (or rented) hardware. Discord simply offered the same good voice chat, but without the server setup hassle or the costs associated with it. The rest is network effects. If your entire gaming guild or clan or group of friends or whatever is using Discord, you are also using Discord. If you mean "why is anyone besides gamer groups using Discord?", that is because most gamers are not just gamers, but also people working in day jobs, or open-source contributors, or active in NGOs, or have non-gamer friends with whom they needed to chat lately due to COVID restrictions, or whatever else. They knew Discord from their gaming life, so they just tried applying the tool that they already knew well to the (slightly different) job. With varying success, but with some success nevertheless in some cases.