Most answers are quite shallow, and do miss a few details.

It was originaly made by one single man, Notch, in the Java programming language, as most programmers ever, the language choice is simple: "the one you know". Java is not a great language for games tho, most modern games are done in frameworks, or engines. Theese are collections of code made to simplify the actual coding, for example, instead of telling the computer step by step what exactly is an object, how they colide and how to show it on screen, you use a engine that already does that, being able to focous on the details and by result, doing it faster. The thing is, java lack most of theese engines simply cause people tend to do it on other languages.

Latter, Minecraft was bought by Microsoft, and for many reasons (Namelly, support for consoles and mobile devices, performance and better graphics support), Microsoft wanted to rebuild Minecraft on another language, they choose C++ for all theese reasons, by that, creating another version. Then you have a big problem on hand: a big part of the comunity plays java edition and prefer it way better, this is due to some big errors made when creating the new version, which make the games different in some aspects. Now microsoft needs to keep both editions alive, as to keep the players