I'd heard that they originally wanted Delaney, or someone like her, for the counterpart to the lead (to keep with the Bill Petersen/Marg Helgenberger formula that works so well in the original CSI) but didn't find anyone they felt was strong enough to play opposite Caruso, so were going to try something a bit different. Then when Delaney's show Philly didn't pan out (and she became available), they quickly moved to get her. Unfortunately, rushing it (and trying to revert to the original formula) proved to not be a good plan for any number of reasons, some of which might be only rumor but fit the facts, FWIW (Possible ego clashes, sudden need to adjust scripts that were already written to include her character, etc.).
Personally, I think that Delaney is plenty qualified to carry a show and the writers (given some time) could adjust the direction in which they were going with her character if they brought her back. It's also a shame about Cochrane's character being killed off as his acting, along with that of Proctor, Rodriquez, Alexander, and Milos (and even Togo, for that matter), are what I have found in the past to be some of the best elements of the show despite the recent trend to key mostly on Caruso's character and use everyone else as window dressing.