Some people will say because the studio monitors are more flat and the surround sound isn't.
For me the surround sound would be fine if every studio in the world had the same surround sound as you, then that would be the standard. But they don't, and the most common standard monitor is probably
the Yamaha NS-10M's, and they're not flat.
By mixing to a known standard, your mixes won't likely be as way off compared to the rest of the world, kind of like the same reason as we all agree on the same tape measure when building houses.
But in reality you can mix through anything as long as you like it, your audience likes it, and you make lots of money.
Here's what helped me:
Go through all the best CD's you can find in the genre you are in. Then when you are mixing, AB your mixes against those recordings. Try to match the volume, the amount of bass, the amount of treble, the ratio of band to lead voice, presence of snare and kick etc...
For me I use this tune called "You're Makin' Me High" by Toni Braxton, as it's really loud and clear. I also compare things to Pink Floyd's "Dark Side of the Moon" as that's an exceptionaly well recorded album.
But pick the best of what you like and try to copy them.
I also have an old Sony boombox that I always try mixes through, and it helps to turn the mix so soft you can barely hear it sometimes as that gives you a good "big picture", or listen to it from the next room.