Because Dropbox just trades in another set of security concerns and potential problems for the ones encountered with Skydrive.
I use Dropbox, I use Google Drive and I use Skydrive. Part of that results from my mixed platform utilization. As an IT professional I work for a 2000 employee organization that is standardized on being a Microsoft shop. We have Exchange, we have Outlook, and we use OneNote extensively for inter and intraoffice collaboration and communication. So, SkyDrive is a natural extension of that which integrates very smoothly with our Microsoft operations.
I write most of my technical blogs on Google Docs, because I am very integrated into Google's ecosystem both with a single web-login for their suite of services and because it all synchronizes nicely with my Android devices.
Finally, I use Dropbox because it is a robust, inexpensive and powerful cross-platform solution for my Windows, Mac, iOS, Android and Linux platform machines.
Microsoft got picked on here because they're the ones I ran afoul of and experienced a cloud based interruption of continuity of access to my critical documents. No cloud service is immune from that potential - and on any platform where you are mixing leisure and professional needs, you run the risk of having those two issues come into conflict with one another. My experience is really just the tip of that potential iceberg.
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