Jack was locally researched in New York.True. I think the problem in New York was that Jack-FM had too many Alternative Rock songs from the 1980s and 90s. NYC never had a successful Alternative Rock station, although several stations tried the format and failed. So WCBS-FM as Jack was playing songs that might have been very familiar in LA, Seattle and Minneapolis that were not familiar to New Yorkers.
The fact is that it was developing much stronger 25-54 demos than the prior "oldies" format. It was successful.
However, the development of the PPM in Philadelphia (the first true test market) showed a strength for a modernized oldies format such as we now call "classic hits". As that format was adjusted in the early PPM tests (around 2002) it revealed that a more 80's based focus with little 60's was going to be a big 25-54 generator. Rather than let someone else grab this format in New York City, CBS went to "Classic Hits" after a relatively brief "Jack" period.
Jack did not fail in New York City. It had, indeed, better 25-54 than the oldies format had at the end and it was still growing when it was cut off. It looked considerably better than the prior "oldies" format which was leaning very old.It was an interesting turn of events. CBS Radio began putting Jack on multiple stations after its initial success in Los Angeles. Many of those former CBS stations still have Jack-FM to this day, still going strong. But others like WCBS-FM NYC and WJMK Chicago saw Jack fail and never return.