There's at least 2 reasons for that:
1. The ratings have been dropping on CSI for years. And that drop got accellerated when they tried to fight with Grey's Anatomy instead of just doing what they do best.
2. In these recent years TPTB have had no idea how to market/promote the show. This season they've spent the first 10 episodes promoting as 'Grissom's leaving' and since then as 'LF is here'. And I've seen stories on LF where he's mentioned he didn't even know he was in the running for the part until he'd gotten it. Until Turn, Turn, Turn they did very little promoting around any of the existing cast and who was going to be staying, and if they wanted a fanbase for after Grissom they should have tried to enlarge the ones they had for the original cast instead of just bringing in someone new.
I don't think it's a surprise that Turn, Turn, Turn did so good in the ratings, and I don't put it down to Taylor Swift being in it (never knew who she was before and she did a decent job for a non-actor). From scanning the web and people's comments about it, it was more to the fact that there was the promise of an episode based on one of the existing, established characters (not Ray Langston) that brought people in.
Susan