I know that testing on IE is possible, but there's more to it than that. If I'm just going to test my stuff on IE every now and then to make sure it works, I'm not going to focus on, or experiment with cool new features the IE teams adds, because the only time I use the browser is when I boot up my IE testing VM.
If IE ran natively on Mac and/or Linux, for instance, I would spend a lot more time actually using it, not just clicking around my app to make sure it isn't broken before a release.
It's usually advisable for people get them from analytics tools on their own sites rather than looking at global stats. For example, IE stats from my Google Analytics account covering multiple different industries, company sizes, and types of sites and applications range from 6.78% to 13.19%, and I've seen similar numbers on every project I've worked on for at least 4 or 5 years.
It is bloody difficult.
Last time I tried using the VMs, the license expired after 3 months, and the VMs didn't work well under VirtualBox/Linux.
Touch support in particular is completely crappy. MSPointer events in IE10, with horrid corner case bugs. Changed to pointer events (no prefix) in IE11, with different bugs. Windows Phone 8.1 update to IE11 supports iOS style ontouchstart events.
Pinch zoom is different from page zoom, and pinch zoom doesn't act the same as iOS/android.
I hate Microsoft's bullshit touch support.
Oh, and about 0.1% (measured) of our users are using IE with touch... What a waste of my time.
I don't think Mac OS/Linux has a huge impact here. And as for the inability to test IE, try modern.ie - free Windows VMs with different IE versions. It's not that difficult.