.NET might get there one day - at last Microsoft are getting off their high horse and talking about supporting other platforms, but its not a reality yet.
Not sure what you're talking about. .NET isn't Windows only, .Net programs can be made to run under Linux through Mono (OSS .Net clone), and Microsoft has now made relevant parts of the .Net framework open source anyways.
I agree. If you develop electronics related software nowadays better make sure to support MAC & Linux as well!
Mac (btw, it's 'Mac' not 'MAC', the name of Apple's computers is not an acronym, and 'MAC' is something completely different than a computer made by Apple!) and Linux are pretty much niche platforms (Macs even more so than Linux) when it comes to T&M applications, fact is that Windows is by far the best supported platform and this is unlikely to change anytime soon.
Plus there's the problem that many (most?) cross platform programs often violate the style guides of at least one platform unless each variant gets an UI that is adapted to the target platform, which means a simple port won't cut it.
If Linux/Mac support is possible without compromising other variants then great but I'd primarily focus on the platform that roughly 95% of users are going to use.