Its written from the ground up and has a clean foundation (which is not the case in vanilla minecraft server code). We also want to use cool modern features like multi-threading or the rust language which is a modern language designed to fix mistakes from older languages
java 1.0 (even 0.9) did have. The 1st language with a memory model was java 5 (1.5), which was back ported to 1.4.2. 1.4(.2) already had non-blocking I/O as well.