I would guess that maybe since you do not have sufficient disk size on your Virtualbox VM (you have only 10GB) to permit a successful side by side installation, so ubiquity quite rightly does not offer that as an installation option.
I would guess that maybe since you do not have sufficient disk size on your Virtualbox VM (you have only 10GB) to permit a successful side by side installation, so ubiquity quite rightly does not offer that as an installation option.