1. To DeDoS a Teamspeak server, it's enough to DeDoS a single server. You may not even need to do that, it may be enough to be such a nuisance that their host kicks them out. To DeDoS a Discord server, it's necessary to DeDoS the entirety of Discord, which is much, much harder, and also much more likely to put you in legal hot water. Discord is the Cloudflare of gaming.
2. Discord servers aren't real servers, they're tenants in an application, effectively "rows in an SQL table", not standalone containers requiring their own tech stack. This means they can be offered for free. You also can't abuse them for E.G. crypto mining, like you can with a VPS where a Teamspeak server can be hosted. Free increases adoption, which makes people a lot more likely to pay for extra features. It's the standard "the rich subsidize the poor" model, common to so many web applications.
3. No technical expertise necessary to set a server up. Bus factor is basically equal to infinity.
4. One service, one account, one interface, many servers, many groups, many people. There's no weird workspace switching and per-workspace DMs like in Slack (not sure how TS does this). If you log in once on a new device, all your server memberships are there, and everything just works. You may be in dozens of servers, and they're all behind the same single login.
Those 4 features are table stakes now, like it or not. If you want to be a real, long-term Discord competitor and attract real users, you have to figure out how to get those 4.