Because a Happ stick needs a little more than 2" of space below the panel...
I bet you that the stick you're looking at isn't even that thick, let alone would it have 2" of free space inside after removing the existing stick...
Most of these sticks & buttons are not anything like the arcade originals... They have PCBs with contact points on them (more like game pads)...
But you WILL NOT be able to put a Happ stick in that case... (well... not without a serious mod & adding a new bottom...)