Disclaimer: I use Ubuntu on a regular basis and quite like Linux as a whole.
Why do you pay $$$ to buy Windows or overpriced macs? Use Linux Mint. It is free and has every software that anyone may need. Or you may like Ubuntu. It also has every software that anyone may need, plus more. You can also try Xubuntu which is every software that anyone may need and it is stable as a rock.
Because for various reasons the software you need to use is only compatible with one or the other (even when taking Wine into account). I need to use Fender Fuse to make my computer talk to my amp because it's been sloppily thrown together in .NET in Silverlight and it doesn't work in Wine. Fender PLUG (an open source project) works to a degree but not very well with my particular model.
Many people need to use Adobe products that are not available in Linux distributions at the moment. To the point that their livelihoods and jobs rely on it.
Countless unique software packages only run on Windows and using them saves thousands (or millions in some cases) a year for businesses. To the point that many large operations (such as cotton gins) rely on maintaining obsolete computers because changing to newer equipment compatible with newer stuff would cost more than any of us will ever earn in a lifetime.
Don't get me wrong, I use Ubuntu, I like what Linux can do, but there is lots that people need that it
can't do because the packages
do not exist. Outright telling people to switch to a Linux distro as a one-size-fits-all approach is foolhardy and, if you are in the IT industry, almost negligent.
Is it good? Yes. Is it stable? Yes. Will it do what most end users need? For the most part. Will it be right for everyone? Of course not.
I don't like Apple products, but if someone came to me and said "I need to use GarageBand and Final Cut because I'm familiar with these approaches and will do exactly what I need to do [x, y, z]" I would say "Cool, buy [at least this level of iMac/Macbook]". I wouldn't tell them "Nah, use Linux and find a package that
should do what you need and spend hours relearning how to work those packages."