I think they simply failed to keep up with the quality they once had.
1) If Microsoft is already/still at ~90%, is there really room for a big time come back? Getting the rest of the market is small potatoes compared to what they already have.
2) How much of the market does desktop represent, anyway? Most people I know do not own a desktop. They may have a laptop, but they may also only have a tablet and/or phone.
Now, I believe you are actually suggesting that Microsoft can leverage their desktop market to get back into tablets and phones, but I don't know . . . so far, their efforts have been disappointing, and frankly, people are used to their Android devices, so Microsoft will actually be the weird phone even if it is the normal desktop.
I think this is the case here.
Many of those devices do allow for a SIM card as well.
http://www.saturn.de/de/category/_tablets-252064.html?search...
TREKSTOR SurfTab duo W1 Volks-Tablet WiFi, Convertible mit 10.1 Zoll, 32 GB Speicher, 2 GB RAM, Windows 10, SchwarzComing back from what?
They've been steady at 90+% market share for a couple decades now. The modest increase in MacOS market share this past decade has never been more than a couple of percents worldwide. A drop in the bucket, really.
Even in the tech world, outside of startups and hipster tech companies, there are a lot of developers using Windows (i.e. the vast majority of .NET developers).
I'm not sure what you mean by Apple failing to make inroads outside of the USA as well. They've made inroads all over the western world.
MacOS has about 1% market share worldwide.
Apple has around 10% [0] of the market for laptop sales (most consumer computers are now laptops, I don't know any non-enthusiasts who have bought a PC in the last 5 years).
Now, factor in that those sales figures include less developed countries as well as developed countries, and you're probably look at at a decent margin about 10% of personal computer sales for home use as Apple products.
[0]: http://www.digitaltrends.com/computing/apple-2015-notebook-m...