For years MS had a near monopoly on drivers. Basically every device manufacturer made a driver for MS and maybe, kind of, sort of, possibly got around to a Mac driver, and then occasionally made a Linux driver. Thus anyone wanting to take on Windows would have had to reverse engineer and make a whole slate of device drivers. As an example, by Mac OS X making the switch to Intel it allowed hardware companies to more easily port their drivers so a few more did.
But over time Linux did managed to do just that, but being open source those drivers are then much more portable to entire other architectures such as ARM. This is then combined with the fact that few people hook devices up to their tablets makes for a near perfect environment to completely overtake the Wintel monopoly on drivers.
So for the first time in decades a consumer does not worry or even know about any driver issues and can choose their device and OS based upon features that are genuinely meaningful to themselves; such as price, app availability, and quality of the hardware.
So with the playing field is now much more level it is not surprising that the former Wintel monopoly is losing market share.
But there is a second and very critical issue and that is of CPU power. Quite simply a Raspberry Pi is around the minimum power that a typical Browser surfing, youtube watching user needs to have. Thus most people don't need the latest and greatest CPU to power their needs. So a halfway good arm inside a device is well enough for the vast majority. Also most people don't need to do much on their computers. A few simple games, some surfing, some video, some messaging. Thus a mobile device is becoming most people's primary portal to the world. Again this does not need to be a powerhouse; it just needs to be reasonably price, work well, and have a good battery life.
But lastly there is the way that ARM is structured. From what I can tell, if you want to buy 10 million arm processors then you buy 10 million arm processors. But if you want to buy 10 million Intel processors then Intel wants to make it complicated and have you enter into a "relationship". The same with the android OS vs the Microsoft OS. Personally I would be very wary dealing with either Intel or MS in that if suddenly my product was somehow incompatible with some corporate vision they had then they would cut me off or otherwise strangle my company. But ARM and Android just want you to buy/use their products.
I suspect that neither of these companies are going to adjust well to actually having competition who aren't even playing the same game meaning that neither Intel or MS will be able to squirrel the rules. Does anyone remember the phase Dell went through where they were Intel only? Can you imagine the angry conversations when Dell, HP, or anyone like that started to ship Linux machines? Do you think that anyone shipping ARM devices even wonders what ARM thinks?