Because the property element syntax
] only applies to properties, not events. It's primarily intended for cases where you can't represent the property value as a string, so you can't use the attribute syntax
Event handlers can only be specified as the handler method name, so there's no need for an element-syntax equivalent for them. Even if there was, the syntax you've proposed isn't valid XML.
bind the button's Command property
] using the property element syntax, although it's not normally required.