I've led startups running Rails and Python/Flask, but I do all my side projects in dotnet. And I haven't had a Windows machine in years, fwiw.
Deployment and Cost: For solo projects, I do 1-click deploys to a load-balanced Azure App Service. I was using GitHub Actions for a while, but it was slow and I kept pushing up against the need for a paid plan. I keep things local.
Dotnet is dramatically more scalable than Rails or Django, which means much simpler infrastructure for a lot longer. Poor performance leads to the need for more complexity. More servers, caching, queueing, etc. Then you need things like docker and k8s to manage all that infrastructure. For me, I can scale up or out by changing a single flag, and entire apps are just single dotnet apps (aside from the DB), so there's no need for docker.
Batteries Included: My team that ran Rails was burned by the magic, repeatedly. Over the years, developers made decisions that turned into maintainability nightmares. With Python/Flask, the lack of structure was like a Wild West after several years of rotating contractors. In some of my older solo-dev work with Flask, it was nearly impossible to come back to because all the disparate dependencies wouldn't play nicely any longer. And in both Ruby and Python, the lack of static typing was also a source of inscrutability (so please use type annotations in your Ruby and Python projects - your future self will thank you).
As for why people don't use it more, I think it's largely due to "resume driven development". It's the same reason every startup thinks they need k8s or they need to be using React. That's not necessarily a bad thing. Choosing dotnet is definitely not going to lead to job opportunities in the startup world. But, in my opinion, any startup that chooses dotnet will have a competitive advantage over those that choose Rails, Node.js, or Python because of simplicity and maintainability.
As for the argument that "time to market is faster with rails or python", I disagree. The biggest factor will be using what you know. The second will be the complexity of your infrastructure, which will be much simpler on dotnet. Avoid analysis paralysis as much as possible.
Other folks may have very different opinions, but as someone who has worked on several major platforms, each for years at a time, I have found dotnet to be the best option. However, no choice will be perfect, and dotnet has many warts. My recommendation if you do choose dotnet, is to go with MVC + HTMX. It's not sexy, but it's stable and mature. I don't recommend Blazor, yet. I think it needs another version. Last note: I've never bought any third party libraries/controls to do anything in my dotnet projects.