I think that today c# and java aren't that far apart. But Scala was created in 2004, c# got lambdas in 2007,and java got them in 2014. So there were a lot of years when the c# java gap was pretty large, which gave Scala lots of time to grow to a critical mass.

I think you're right another factor is that there are more Python and Ruby devs looking to write Python and Ruby on the JVM then the clr.