The "point" from a business stand point should be to make money. Why would you release anything specifically targeting some tiny demographic of the overall consumer base unless you have exhausted all other higher potential income streams and the projected profits will be less than the cost of production?
Well, you wouldn't. But why not develop your first client using an open platform like HTML5 such that it works on every operating system whether proprietary, free, desktop or mobile? Deliberately limiting your userbase to just those using Windows seems to me like leaving money on the table.
I'm not saying "develop only for my_niche_os instead of for Windows".
I am saying "develop for every platform, not just one of them".
You'll probably find the stats are different for bitcoin users, but Windows may still be the most used. I'm not saying it isn't a popular choice. I'm saying it is a poor choice. Lots of people eat at McDonalds and smoke cigarettes but that doesn't make those great choices either.
As far as safety goes, the end user can secure a windows OS machine just as well as anything else.
That's not true. It's almost impossible to even know what a Windows machine is doing. Everything's closed source, delivered in binary form only. It takes an awful lot of work to reserve-engineer that stuff to find out what it's doing. And then Microsoft get to install new binaries on 'your' machine whenever they like via their update system. So you really have no idea what's going on. Good luck securing that!
The tools and information are all out there. If someone has their computer hacked or bitcoin/information stolen, it is likely the fault of the user and not the operating system.
It's always the user's fault in the end. The user decides which OS to run after all. It's not the cigarette's fault that I die of lung cancer; it's my fault for choosing to smoke it.