Well as a huge tablet person, someone who follows the market really close, who has used many OSes and many form factors, I would say the reason is very obvious.
Windows 10 on a tablet, is full windows 10. Android on a tablet, is a big smartphone.
You can run win32 apps on a tablet, you can real-time multi-task, snap windows, run a full featured browser with html5 webapps and extensions. Even for the average consumer, the desktop OS windows 10 offers noteable "power of software and function of OS" advantages over android.
Windows is the fastest growing segment of tablets, the only other one apart from budget devices. Samsung and apple are both losing growth and have been for years now.
The surface with you point out is lost growth, _this last quarter_ is two years old and was only made to lead the way for the other, honestly a lot more affordable contributions from their hardware partners like Lenovo (and many others), for whom business is fantastic. The surface is also pretty pricey given current market conditions. Even apple released a midteir ipad recently.
There should be no deep reflection on the surface line, which by MS own words are only designed to lead the way for their hardware partners - which is exactly what has happened.
A windows tablet can do a lot more than either an android device, or windows 10 mobile. Its a full PC. It can run the 12 million or so win32 apps, which are generally much better funded development wise than mobile apps are. And I run plenty of them on my windows tablet, with scaled up UI, they generally work well. I've never felt any need for a stylus, or found them awkward.
Occasionally they may scale incorrectly at worst. That's not an insurmountable issue. And its actually pretty rare and can fixed by compatibility settings (which have been expanded quite a bit).
People will spend over a hundred dollars on a windows game. They will spend hundreds on professional databasing software, music composition software, video editing software and so on. Even regular users will happily shell out more money for desktop software.
People are relunctant to spend 4 dollars for an android or ios app. And that huge magnitude of development money, is why you get deep and powerful software for desktop, but only puddle deep applications on mobile.
And the feature set, such as real-time multi-tasking and optimisation for more powerful software, and also a vast library of drivers for connected peripherals. You can for example, easily use your tablet as a full desktop - plug in anything you like, and it will work. I've run a full office set-up with my tablet, printer and scanner, external webcam etc etc.
The driver compatibility of android is pretty poor. It scales poorly to larger screens, and the software is 99% written for smartphones making the experience largely pretty poor, overly simple and smartphone like. By and large things scale down, better than they scale up.
The app gap, really here is larger for android. Nobody really writes for tablet in android. Whereas in UWPs tablet is pretty strong, and for win32's, anything larger than 9 inches (under which you can use a stylus) , win32's work well. And those win32 are also getting converted into UWAs, for example there's a great video editing suite in the windows store.
The area UWPs are growing the most in, is areas specific to tablet. For example, an art program - you'll find AAA leading examples of art programs in the windows store. There's not really anything in android to match. UWPs suffer under mobile, in mobile specific applications - gps databasing tools, messaging and so on. Outside of those applications, the windows store is growing fast.
If you want to just use your tablet for watching Netflix, and looking at a browser, android is a good choice.
But if you have any kind of productivity interests, or power user leanings, the difference between the platforms is pretty night and day. And windows 10 has some features to intergrate with android these days, if you go for an android phone. More than just contact syncing, things like notification syncing.
There is also no real sense in which an android tablet, and android phone "pair". They aren't designed for that kind of use, they more or less operate as separate entities. The services are shared, but you wont get for example notifications for one device on the other. Or the ability to continue editing a document from one device to another. These are really MS domains.
Interopability and continuum etc are really windows specific tools.
Tablets are absolutely _not_ a "loser for ms", not even in the ballpark, they are a huge success for MS, and MS is investing even more in them with a new surface expected this year, and the release of windows on arm, and windows cloud. MS has realistic ambitions to take over the tablet space.
This is what I have learnt by working with these devices, both android and windows on tablet. I don't even really see them as comparable. I see a windows tablet as a PC, and an android tablet as a big smartphone.
It's pretty hard to know what to recommend in a tablet though without knowing a lot more about your useage habits. Do you travel a lot. Do you generally carry a bag when you do. Do you type a lot, or write emails, blogs etc. Do you make a lot of paper notes. Are you the type of person to use a device for a shorter period, or intensively for hours. Would you read a book on one, or a magazine. What kind of software do you use on PC, that you might use on a desktop. Ditto with a smartphone.....
Even I were to say "always go with windows" (Which I actually might baring a few specific exceptions for those who "simply don't have the need") there are a lot of different forms and sizes of tablet, and they all have their own strengths. It takes a person using a lot of different kinds, to figure out on their own what works.
Coming from the other end of MS, desktop, notebook and tablet, I see a thriving company with a energetic and strategic vision for the future.
And actually I see a commitment to mobile that I doubt any other company would have replicated in the circumstances. Android OEMs and google don't give feature updates to older phones. Blackberry dumped bb10 with zero feature updates literally one year after the release of some bb10 phones. From where I am standing, as someone who's been involved in mobile devices in a broader sense, MSs stance on a dwindling mobile market and a mobile company who generated massive losses, seems "generous" (although really its not, its just part of a more long term play to unify devices and try and continue to expand in to new spaces, mobile included)
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