Monks are the least played for two reasons:
They're not very competitive.
They're difficult to play in all specs. They don't require a huge amount of theory crafting, they just have a lot more buttons to push compared to some more 2-dimensional classes.
I sincerely hope they don't dumb them down to increase representation...but those numbers are just depressing. Only class to dip below 5%...
Also, what's with the marked decrease in Hunters? That's a pretty sharp drop (3%)