Mostly because the alternatives disappointed me.
Tablets: Tried the iPad, went through tons of Android tablets... they were all... little toy OS tabs. Surface tops them all in a way that makes it more versatile than the Macbook it's so commonly compared to, any of those media consumption tablets... or any laptop for that matter.
Console Gaming: After a big blowout with me and Microsoft's trigger-happy billing dept, I'd gone to PS3... sunk in a lot of money to ultimately end up bored with it... it was always just "that system without Left4Dead" to me. I'd sit on the sideline enviously watching as the other end of the battlefront got Lost Odyssey, got countless versions of fighters that had more reliable netcode and better communities... then even have my fires stomped out when PS3's Vesperia got canned. After learning that I could make XBL Gold my ***** with the plentiful offers of low-cost 1yr subs, I found myself back on the winning team.
Media: I love when Charter calls trying to offer me their stupid cable package because at the end of the day... no one that understands Windows 8 will pay for cable. A little $60 N281 has been my "Roku" and I pretty much get to watch whatever I want from sources that are virtually endless. W8 is beautiful on the big screen, and it plays well with air mouse remotes.
Home Computing: I'm always open to the idea of trying out something new... but the moment I have to add on a virtualization layer to get Windows, that's pretty much admitting the alternative blows. Everyone with a job that involves computers has pretty much admitted their Mac is a multi-thousand dollar brick until it's Bootcamped. Linux scores hella geek cred with the neckbearded contrarians, but the end result of Linux is a bunch of distro fragmentation, dependencies that make Linux needier than the neediest high school girl you know... and a whole ****ing lot of terminal. I've also found Linux communities to have the worst forms of elitists...
Smartphone: This was moreso just completing the ecosystem more than anything. My PC, laptop, tablet, game console were all Windows, so I figure it was time to rid my life of force closes and make WP my daily driver. I wanna say the ecosystem has catching up to do, but honestly I can't find one thing that I did with my iPhone/Android that I just can't on my WP...and knowing that alternatives to what I used exist allowed me to appreciate a lot about Windows Phone. It gets a fair bit of customization putting it ahead of Apple's dinosaurUI, and what it lacks in customization compared to Android, it has in reliability.