Because you or a team knows it, because it's still a hell of a lot more of a batteries included ecosystem than the last Scala or node projects I was involved in which had to reinvent a shocking number of wheels that are table stakes in Rails-land (can't speak to .net, never been an MS stack dev), because you're working with some other Ruby or Rails thing and its easiest to stay compatible, because there's a lot of mature, profitable projects out there hiring in it with high salaries ;), etc

It's an engineering decision with tradeoffs, like every engineering decision.