Basically, there was Bukkit. In 2011, Mojang hired a bunch of Bukkit's developers, but didn't tell anyone that they also legally took control of the Bukkit project. This was because Mojang didn't really want Bukkit to be their official API as much as being a temporary API for the time being (and also didn't want to have anyone assume they would be responsible for it), but from that point, Bukkit was legally their project and the community didn't know.
Then, Mojang changed their EULA to ban Pay-to-win servers. This caused a lot of controversy, and one of the Bukkit developers left. Mojang then revealed that, yeah, they owned Bukkit for years, so they didn't need him. Another developer, feeling betrayed, who had contributed to Bukkit, submitted a DMCA takedown against his own code bringing Bukkit down with it, angry at having done "free labor." (You might say, "but the GPL!," but being tricked into a contract under false pretenses can legally invalidate contracts, making the GPL-license for his contributions potentially invalid.) Mojang panicked at the takedown and a ton of developers abandoned ship, working on Bukkit's community fork Spigot. Mojang then 2.5 months later brought Bukkit back and that time really just threw it into the community's hands.
The result of the aftermath was everyone had restructured around Spigot, based on Bukkit; and Mojang wasn't going to deal with APIs and open-source stuff for a long while afterwards. (They did later try purchasing parts of OptiFine, but the developer wanted compatibility to be all-or-nothing which Mojang wouldn't accept.)
https://www.reddit.com/r/Minecraft/comments/6g1tqn/psa_no_bu...
https://blog.jwf.io/2020/04/open-source-minecraft-bukkit-gpl...
https://www.reddit.com/r/Minecraft/comments/2fk5nn/my_respon...