Why everyone loves Garageband for Windows users
Some Apple software has been ported from OSX to windows, like iTunes and QuickTime, though typically only when there has been a targeted business case for doing it. In the case of iTunes, to persuade users to purchase Apple hardware (iPod, iPad, iPhone etc) without needing an Apple computer to use it.
Garageband for PC
Proprietary is very much the Apple business model and once you take a dip into the Apple ecosystem, it tries its level best to imprison you in it, with breaking updates to software requiring new hardware purchases, convoluted methods for transferring content to your i-device and built in obsolescence that would make a replicant wince.
Garage band is easy
While I’m obviously not a big fanboy for Apple, back in 2004 when Garage band was released, an uncle with similar musical tastes brought his newly purchased MacBook pro, with Garage band installed on it, to Christmas at my parents place and I found the user experience immediately compelling. Within minutes I was using samples, loops and musical keyboard to create music having never used a Mac before. I think that is the single biggest reason Garage band is so huge, it really does help you to create.