montynut, It basically comes back to the concept that its all about the "short travel era", not specifically 31st of December 1974.
For whatever reasons, it became 'pre-75', which has since revealed a few weaknesses - including the GP Maicos and YZ-Bs. Personally, I reckon that those bikes should never have been allowed in Pre-75 as they are clearly the first bikes of the new long(er) travel era regardless of their date of release.
The same thing applies for pre-78, but even more clearly. The Pre-78 class is specifically about the bikes that had more than 7/4", but less than the ~12" that bikes subsequently settled on - and yet a significant number of 1977 bikes simply don't meet that criteria.
The regs as they stand are a mis-mash of two idealogies - the one that says "technology is what really matters, and the one that says "the year of manufacture matters" - without actually satisfying either.
IMHO, if we could go back in time, we'd be far better served by rules that followed either idealogy. But I'm equally sure that the cost of ditching the current system and adopting either idealogy now would far outweigh the gains...