The "original coders" probably didn't decide anything, they were given a spec sheet driven by the original client's requirements that said you must be able to enter a first name and a surname.
The reason such forms have two name fields rather than one is fairly obvious, which I'm sure you are aware of.
Ask yourself, how many people do you know, in real life, who just have a single name. In 40 years I've never met one, and only once spoken to one on the telephone (he changed his name to "Tank", no surname and no first name, just "Tank")