Lack of skill is one reason!
I haven't even tried to do a burn out yet.
Practice when you're about to change tires.
But if you feel you're about to screw up, pull the clutch.
If you only close the throttle the back wheel will stop and it can get squirrely real quick.
If you panic, you'll likely go down, or fire the bike into the air...I've seen both happen.
It's hilarious, and expensive.
i'll be doing that in about a couple of weeks because i need a new rear before my track day. the "standing burnout" is not that hard let alone a "rolling" one. stand up, lean your gut into the tank, push on the bars, grab a monsta' full of front brake & wrap the throttle with your palm. results: instant supa star!