For the same reason I wouldn't just haul off and bust the guy up, if it wasn't warranted. The drunk idiot who is, well, being an idiot, might not need his arm broke, eyes gouged, or teeth knocked out if he simply puts his hand on you, when a simple controlling method would work. Assess the situations accordingly. If the situation warrants busting the guy up, then do so.
A drunk isn't getting me to the ground period. I have handled drunks for many years.....When I worked the doors you had legal ramifications to consider as I still do when I do an event security job. In that situation I go open hand all the way. Big hits from a fist don't look good on cameras in a court room