As full stack .net dev, I am so disappointed every “who is hiring” thread. I would be game to join a trendier group of like minded devs but if you get one .net posting you are lucky, and it’s most likely rockstar.
I think an issue here is that .NET was closed source for most of its existence. It was only 2019 when Microsoft announced that the open-source .NET would be the future of .NET. That will mean that a lot of startups wouldn't have chosen it. Even now, it means that a lot of the "trendier" devs will have learned other ecosystems that have been open source for longer.
It was 2016 when you count official releases, open sourcing .NET to MIT started in 2014 including huge parts of the .NET Framework.
It is soon 10 years. With 14 years of closed source before. But honestly, it is not .NET which is the problem maker, it is Visual Studio on Windows. Because that is the thing which cost and the thing which only runs on Windows. I hope they fix it in favor of VS Code (which looks good right now).
More important than being open source was the fact that it was Windows only for the first 15-ish years of its existence.
Mono never got traction within the startup culture. In part, because it operated in a legal grey area for many years regarding MS patents for web (or web adjacent) technologies like ASP.NET and ADO.NET.
Microsoft made some extremely generous patent grants to Mono and several major Linux Distributions relatively early in Mono's history. Microsoft didn't seem to want to repeat the mistakes that led to their divorce with Java and the early foundations of .NET with their relationship with Mono. In my experience of it, most of the "legal grey area" concerns around Mono for a lot of its history was either truly outdated/ignorant advice or intentional FUD from antagonists (many of which just hated anything Microsoft indirectly touches no matter what).
I think it depends on who you are. In 2007, a lot of people distrusted Microsoft and with good reason. They'd spent a lot of time extinguishing competitors. .NET Core was initially released in 2016, but Microsoft wasn't saying that .NET Core was their plan for the future. It seemed like an experiment they'd likely kill off or that it might just be a small subset of .NET.
I think saying that Microsoft started open sourcing .NET in 2007 feels a bit disingenuous. Plus, wasn't it source-available under the Microsoft Reference License? Regardless, .NET was still tied to Windows unless you wanted to use Mono (which was slow and had an uncertain future).
If you were someone who developed on Mac or Linux and deployed to Linux, you couldn't choose .NET in 2007 or 2014. Even in 2016, are you going to choose a brand-new ASP.NET Core? Microsoft announced .NET Core 1.0 at the Red Hat Summit in 2016 which isn't exactly an endorsement that the company thought it was the future of .NET. It seemed like Microsoft was trying to open up just enough to hook a company on .NET Core and then tell them "well actually, you should really upgrade to the real .NET Framework on Windows" when they ran into problems. That's not what happened, but Microsoft certainly hadn't committed to .NET Core in 2016. Their messaging was "well, Red Hat will offer support for this thing we made."
If I didn't have a Windows PC in 2015, I couldn't do .NET development (Mono aside). If I wanted to deploy to Linux in 2015, I couldn't do .NET development (Mono aside). The open source part matters to lots of people because .NET simply wasn't a choice for those who didn't want to be beholden to Windows and licensing until recently. People definitely ignored .NET because it simply wasn't an option for them.
My feeling was that Microsoft open sourced the .net framework to help Mono catchup a bit.
As for access to windows, to be honest I think that's pretty easy to have. Run on general purpose hardware, it's not that expensive.
I think that having access to a Mac is relatively harder than having access to a Windows machine.
When .net core came out I was under the impression that the product was the future of .net but maybe I was living in a bubble.
Anyway, I understand your point but I still think that the disinterest towards .net is tied to a generalized adversion to Microsoft (probably deserved) but not really related to the tech itself nor it's availability on Linux or Mac.