If the cerebellum is working efficiently then learnt skills will be automatic and will not require mental processing or effort. Therefore attention / concentration can be reserved for processing information and making decisions. If the cerebellum is not working efficiently then the attention and concentration is used up trying to perform the skill (that should be automated) and none is available for the task in hand.
For example. If the cerebellum is functioning efficiently then during a writing exercise the grip and control of the pen, formation of the letters, remembering the correct spellings, etc. will be automatic (as this is a skill that has been learnt and automated by the cerebellum). Therefore the remaining concentration can be used to structure the thoughts in the head and write them down.
If the skills are not automated then all of the attention will be required to physically write and work out how the words are spelt, etc. Only limited capacity is therefore available to structure thoughts and this is why people who struggle often get tired quickly, loose concentration and struggle to stay on task.