I'll second the responses by the others. I'll add that in situations where some of the specs on a given part may be "superior" to those of a competitive part, it will often be the case that both specs are sufficiently good that the difference is sonically irrelevant.
As to why Esoteric chose the Asahi Kasei part instead of an ESS Sabre part, I wouldn't begin to want to speculate. A quick scroll through this 42 page datasheet
for the AK part will give you some idea of the complexities that are involved. (ESS doesn't appear to provide detailed datasheets at its website). Also, I would expect that experimental evaluations were likely to have been performed, perhaps using circuit boards provided by AKM, ESS, etc. for evaluation purposes. And non-technical factors may also have been involved, perhaps including things like working relationships among the particular companies, and how much familiarity and experience Esoteric's circuit designers may have with the products of the respective chip manufacturers. Perhaps it is even relevant that AKM is a Japanese company (as is Esoteric), while ESS is headquartered in California and does a lot of their work in Canada.
In general a great many things, both technical and non-technical, can factor into the selection of a key part in a design, and I would not want to speculate in any given case.