ORIGINAL: BMatthews
because the Center of Pressure concept went into the toilet for the specific lift value of CL=0 where the CP was located infinitly far behind the trailing edge. Mathematician's may not mind infinities in their equations but engineers hate them. Hence they came up with the idea of a consistent pivot point and the pitching moment which avoided the whole irritating infinity issue.
That decision has always seemed somewhat humorous to me.
If you think about it, why exactly would it matter where no lift was. Zero lift produces no pitching moment after all. So why would it matter where the zero value was.
What did seem true about their being able to arbitrarily decide "where" showed how little value there was in their proofs.