It's because FiiO offers great value for the price.
You'll have to pay much, much more money to get something significantly audibly better than an E17 or so. Four-figure easily. And even then you might prefer the sound of the cheaper devices since sound perception is highly subjective, especially in Hi-Fi / high-end scope.
Also, don't let yourself be fooled with all that virtual surround sound stuff. It's can be helpful for competitive purposes imo, i. e. localizing enemies in shooters and such. But in the end it's still stereo sound that comes out of your headphones, altered by some proprietary psychoacoustic software algorithms at the cost of audio fidelity / quality in most cases. On the top of that, it doesn't even work for some people as I've heard. They're just immune to the effect.
Please do yourself a favor and go for pure stereo. That'll save you a lot of time, headaches and even money provided you're crazy enough about audio fidelity / quality.
You may know it already, but just in case. I highly recommend visiting head-fi.org, in particular this guide. That place is the best resource for headphone related stuff out there.
http://www.head-fi.org/t/534479/mad...uide-5-31-2015-beyer-cop-and-shure-1540-added