Murder and Manslaughter in England and Wales; Murder and Culpable Homicide in Scotland. Manslaughter and CH are broadly similar, though not an exact match.
Murder generally must be premeditated. It involves to things: the actus reus, which is the actual committal of the act; and the mens rea, which is the thought in your mind that you actually want to kill the person.
In extreme circumstances a wilful disregard for someone's life 'might' constitute the mens rea, but in many cases would be hard to get past a court. Without the mens rea, therefore, you fall back on culpable homicide, which still requires a mental state of actively wishing to cause harm, or acting in a manner that was likely to cause harm.
The difficuly with deaths caused by driving is probably twofold. Firstly there are specific statutory offences (i.e. mandated by law issued by parliament) such as causing death by dangerous driving, and so the first thought whenever someone is killed on the roads is to use this codified law, which tends to have lesser sanctions than the 'common law' (murder and CH have been defined and amended over the years by court judgements, not what parliament says). Secondly, and it's tied in with the first point, it would likely be a lot harder to argue a case of CH in a court simply because proving the requisite lack of mental deliberation would be hard.
As has been said, very very very few people get into a car every day with the deliberate intention to hurt someone else on the road. So we have to look to the actions to see if they show a wilful disregard. Negligence, even extreme negligence, isn't necessarily such a wilful disregard, and each case would turn on the particular facts. Speeding, for example, on a motorway at 4am, you wouldn't expect to hit a cyclist - there is a chance, but a tiny one; speeding through a pedestrianised shopping area (to use a deliberate extreme) likely would show the necessary disregard.
So rather than run the risk of a court giving a not guilty verdict to a state of mind which you have to prove, the codified driving offences are much more likely to obtain a conviction.
Of course there's then the argument that often someone is killed, the driver is charged with causing death by dd, but negotiates it down to dangerous driving on its own, or causing death by careless driving (a relatively new offence), which are both obviously lesser offences, quite often not even resulting in a driving ban.