Mass Storage devices are attached as block devices. MTP devices are attached as file devices. As long as the OS understands the filesystem on a block device, it can mount it. For file devices, it needs to speak the protocol (in this case MTP). MTP is similar to how Apple handles attaching of its devices, just a different protocol.

Mass devices can be easily corrupted and have to be unmounted from the current device. MTP can stay mounted and accessible by the mounting OS and source device simultaneously and are much more difficult to corrupt.

It also allows the host device more security and control over what the guest can do.