That would be because that device is running an older Android version that does not allow for making the SD card "Internal" storage rather than the standard "External" storage, which is what it would need to do to be able to move most apps. Only "Marshmallow" allows for that option.
You can move photos, images, videos, music, and some other data, but not most apps.
You can try and see if your tablet can be updated to "Marshmallow", but that is not possible with all devices.
If you cannot upgrade to "Marshmallow", or you can but it still doesn't work (some devices just won't let it), then there isn't much else you can do.
Side Note: Even if you can get it to upgrade, and let you make the changes, you will have to start with a blank...