The usual reason given is lack of libraries compared to python. I started a project a while ago with the idea that you could create a ruby library that would implement the C python extension API using the ruby C extension library... I got some basic stuff working, so you could "import" a basic python C extension in ruby (after recompiling it against my headers/library). Then you can use a python -> ruby transpiler (which does exist) to convert the python parts of the code to ruby and maybe have stuff "just work". The more I got into it though, it seemed pretty daunting to get something like numpy working... so I have stopped for now.