Main reason not to is that it increases your odds of taking a beautiful creature and making it suffer a long slow death when you could have used something appropriate and reduced it to possession and fed your family and friends with it for a good part of a year. Yes a .223 can kill an elk if everything goes right. So can a .22 long rifle. But why take that chance? Just to show off? Bragging rights? It doesn't seem wise or ethical to me, legalities aside.
