Because inheritance is broken on MouseEventCtrls.
Yeah I've noticed it can be unreliable unless it is done a certain way
I got it to work in TMBI for a couple of mouse event controls
basically the mouse event control has to have the name when it is created, which means that it looks like it doesnt work in the gui editor until you name it, then do control+x and then paste it back in. However, as long as it starts with a name in script, inheritence works correctly.
That befuddled me for quite some time