I suppose it is for the same reason that people often find it more attractive to buy a car with a three year warranty rather than a car with a one year warranty. The higher standard is perceived as likely to cause less trouble in the short to medium term.
An airline wants everything it buys ( including its labour) to be as reliable as possible. In the case of medicals, if that company is prepared to invest in its own investigations then it has the right to expect a good return from that investment. Many companies do not, and simply rely on the JAA medical standard. That may be perfectly acceptable of course and presumably saves some cost and they accept the standard.
As a footnote it is worth mentioning that the medical standards set for initial issue of a Class 1 certificate should reasonably reflect the fact that the certified individual has been assesed as fit to the level that an Airline ( or employer) could on a balance of averages expect a reasonable length of service from the holder.
Of course there can be no guarantees, but any condition that exists at the point of initial assesment that either fails the standard set or is likely to deteriorate, might reasonably result in the invidual failing to meet future standards for medical renewal at a much earlier age than the median. As dissapointing as this is, it is a case of striking a balance between what is fair to the candidate but also and perhaps more importantly what is fair to the future potential employer. Medical authorities always asess each individual case and naturally there will be mistakes and borderline disputes on occaisions, however the standards set reflect those of a reasonably healthy individual. They also reflect the fact that the candidates attainment numbers will naturally deteriorate with advancing age.