To add why MS is failing in mobile... is their own doing.
Go way back to the old days... 2007 MS releases Windows Mobile6.0 (I'll use the WP moniker for simplicity) for which there are a few brands. iPhone is new and is killing them, easily. As MS/Ballmer said he iPhone was a toy/fad whatever. By 2008 Google's Android is on the market and quickly starts gaining traction. Over the next two years or so, MS slowly releases updates. But its really nothing much more than a touch-screen feature phone (no app store). In 2009~2010, MS buys Danger Phone company and come out with the MS KIN One and KIN Two phones - why they are well made phone - kind of like a cross between a cheaper BlackBerry and Pre-Metro UI - nobody buys them.
Now, all these products WP6.x are Windows-CE based. KIN's OS is also Windows CE. They are NOT cross compatible and do not have a store for more apps. Less than two months on the market, MS pull KIN out and repackages them as Feature phones without the social-network functions (which were actually well done). The KIN screwup costs MS $1Billion dollars and they didn't get sharks with lazers on their heads!
A few months after the death of KIN (but still being sold/used) in the fall of 2010 is when MS released WP7 devices. WP7 is also based on Windows CE and has no compatibility or relationship to WP6.x or KIN. Roll out was kind of slow... sales were slower. Missing were basic features that were expected and included on Android and iOS. But at least they had a WP Store this time! MS never really gets to 1% marketshare with WP7. All support for WP6 and KIN is dead (SONY and others are not happy)
What didn't help is that Apple ships out the iPad and iPhone4 which totally steals whatever thunder MS had with WindowsPhone7. Apple pretty much sold more iPhones in a day that all WP7 sold combined.
By the time WP 7.1 comes out... we learn that WP8 will be part of Windows 8 and WILL not be compatible with WP 7.x hardware nor will the apps be cross compatible. While this seems trivial for a WP7-8 user (simply download an updated app) - its not so simple for the developer who has to RE-CODE the app for a totally different OS which has a tiny market share as it is! Imagine coding for Windows 95 then re-doing your work for Windows 7 64bit. When MS released WP8, support for WP7 pretty much died overnight.
And as we know, MS did a back-room deal and bought out NOKIA mobile (not the whole company) for $7.2 Billion and killed anything that wasn't Lumia/WP8. Not much incentive for HTC, Samsung or anyone else to make WP8 phones, eh? Not like they sold well with WP7 anyway.
Also, Nokia had the N9 Meego phone, which was dead before it was released (Nokia said it would be the ONLY Meego phone, in favor of WP7). The N9 looks pretty much exactly what the Lumia phones are. The Nokia N9 won awards and rave reviews - but not a recommended buy.
If Windows 8 (desktop OS) was a success that people loved, then sales for the Lumia/WP8 phones might have taken off. That was the plan... and we see how that turned out.
Fast Forward today... WP8/Lumia phones are barely selling as they are in the back corner of the carrier stores or bottom end pay-as-you-go phones at Walmart for $70... which compete against bottom end Android phones from LG and Samsung.
Add to this mess, early 2014 - MS/Nokia changed the Asha line (a Linux bottom end OS, weak version of MeeGo put into bottom in Lumia-style bodies) to the Nokia X platform (again, Lumia bodies). This was a cheap phone in which they forked a version of Android, ripped out the google services and bolted on their own Nokia Store. Effectivly making a mostly useless Android phone with a UI that is an much uglier messed up version of Metro from WP7! If you manage to get the Google Play Store installed on the phone, it would brick the phone on purpose.
After a few months on the market, MS yanked this bastard off the market... as they simply made a very plain WP8 Lumia for the bottom end users.
What did MS buy for $7Billion? A possible Android competitor. Nokia could have kicked ass with an Android platform. But you see, MS didn't BUY the whole company, nor its software. They bought the development team, the trademarks (temp), production and a building or two. Pretty much ALL of that is gone. They cannot use "nokia" anymore. But they do own the Lumia name. (WOW! $7B) The phones are made in China like everything else... the OS is made in-house... There is pretty much nothing LEFT of the Nokia we once knew. But wait... In 2016, Nokia will be allowed to sell NOKIA branded phones. Now, they won't actually be built by Nokia since they no longer have the phone factories. They'll do what Apple, MS, HP and others do... design the phone, have it built in China. And you can bet they will be Android phones and they will sell.
The Future: Latest Lumia phones 640 & 640XL series now look like Apple iPhone 5C. Really, they do (see above). The model numbers don't make sense and of course "nokia' name is gone. The new models should be more "cool" like "Lumia V2" The "new" model numbers may seem like OLD models next to the OLD 800, 900 and 1000 series... dumb again, Microsoft.
MS has always failed in the Mobile market. They continue to fail in the mobile market.
Steve Ballmer and many major VPs were fired (Xbox, Windows, mobile) in the past two years.
There is reasons why not many companies develop for Microsoft.
We don't need them... and not worth doing business with, even if it takes 10min to recompile Android code. They only make a profit from Windows OS and MS Office, that's it. Every Xbone is sold at a loss... its about twice as big as the PS4 and very ugly. Only a few exclusives and those are microsoft's such as HALO.
How about Windows 10? Its a FREE upgrade. When has MS ever done a FREE OS UPGRADE. They need it to be a success. Their future depends on it. Its primary job is the launcher for MS-Office. But we don't know what they are going to do after the first year... Maybe they'll come out with version 10.1 and kill support for Win10.0 (refer to Win8 vs 8.1 situation) with the demand for the upgrade for the security patches... then eventually go into a subscription model. They say its always free... but would they put that in ink?
Check back here in 6 or 12 months and I bet their mobile sales will be down... unless, they knock it out of the ballpark with Windows 10. (which, IMHO - is just as downright ugly as Windows 8 - give me a Win7 Skin and I'll give it a try)