Basically MAAS relies on everything it creates to be blank machines which network boot into an installer that gets some image and config dumped onto it and then reboot onto the installed system.
Because containers by design cannot be netbooted and so need to be created from a premade image, MAAS doesn’t really know how to do that. MAAS effectively uses LXD as a more tightly integrated alternative to libvirt and only makes use of it as a way to divide a physical machine into a number of VMs which then are themselves managed by MAAS just as it would physical machines.