In most cases it simply comes down to lack of regular maintenance. Like any other machine, cars, vans, lorries, aircraft, even push bikes need maintaining and depending on usage and age this maintenance should be carried out at regular intervals.

An annual service can and does reveal either current or up and coming problems. Spotting potential problems early in most cases saves money as the cost of putting the problem right can be expensive, coupled with the fact that should the car fail it's MOT, by law the car can't be driven away from the MOT station as it is illegal to take the failed vehicle on a public road. So, by rights, if you want to take the vehicle back home or to a repair shop, it should be transported not driven.  

When a vehicle fails it's MOT to prevent you having to pay for another MOT test, as long as the vehicle is fixed and returned to that MOT station for a retest within 10 days, the retest is free.