Part of the issue is satisfactorily proving 'intent'. Punch someone in the chops, your intent was pretty clear - you intended to give them the old 5 finger poke. Running someone over, without some other decent nugget of information to prove intent, is not so clear.
You can recklessly assault someone, ie, be doing something so daft it seems obvious to any normal person that it could end with someone getting hurt. For example, randomly throwing a brick in public - you may not be intending or even wanting to have it smack someone in the face, but its pretty obvious to anyone with a braincell that is a likely outcome. That would be an assault.
However, get reckless (or careless) behind the wheel and that falls firmly within the realms of a motoring matter, and moves away from assault. If intent can't be proven (be aware thet knowing something and being able to prove it beyond reasonable doubt to a Court are different things entirely) , the dibble have to rely on recklessness, and where a vehicle is involved there is a specific set of laws for that
Im not saying I agree with it - the law as it stands is long overdue an overhaul on this one. However, because of the above I do have sympathy with the prosecuting agencies, because unless that specific intent to assault a person can be solidly and unchallengably proven then the flowchart moves them towards the motoring offences whether they like it or not.