Being to aggressive in breaking stuff gets you a shitshow like Node.js or Ruby. Long-term source code compatibility is a very useful feature for open source and a sign of a mature eco system. Feel free to add stuff, but once it's part of a stable release it has to be maintained long after a "better" way to do it comes along.

nodejs itself doesn't have very many breakages; i have plenty of code that is unchanged from 0.12 to 24. npm is a whole other kettle of fish but I don't think you can blame the core project for the sins of everyone that publishes to the package manager. Python2 -> Python3 on the other hand had a lot of breakage in "standard" code.