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.
My gut feel is that the .Net ecosystem is probably more complete than the Rails ecosystem, but part of that is because there's a larger market for commercial components. Like you say, tradeoffs.
It's an engineering decision with tradeoffs, like every engineering decision.