You could have asked this question anybody who tried to write SciPy or NumPy before they existed: "if you're doing science, why would you want to use python over fortran?" Or, befor ROR existed, "if you're doing web development, why would you want to use ruby over java/perl?". Your question is basically asking why anybody would ever try something outside of an established ecosystem.
The reason is, they like ruby so much that they want to use it for number crunching too, and not to have to use a less appealing language. They want to build an ecosystem so other people can join and contribute and grow together and maybe outgrow python. They want ruby to win so much that they are willing to work on duplicating a framework existing somwhere else.
The reason is, they like ruby so much that they want to use it for number crunching too, and not to have to use a less appealing language. They want to build an ecosystem so other people can join and contribute and grow together and maybe outgrow python. They want ruby to win so much that they are willing to work on duplicating a framework existing somwhere else.