For how it got so big, after it took over the gaming market initially it's likely network effect in action.

Discord is a centralised IM + basic forum with commercial polish.

Small communities can't afford site hosting and moderation, FOSS alternatives like Matrix are significantly inferior products. Fandom killed independent wikis, Reddit killed independent forums.

If Discord ever goes down, there will be decentralised services competing and advocating freedom until a new centralised service takes all the users for itself, just like Mastodon and Bluesky.