When you tell a story you need a push and pull. With Life on Mars, the push is Sam wanting to go back to the future and the pull is wanting to stay in 1973. We know why he wants to go back to the future, we understand that push, but as of now, there's no believable reason to believe the pull of wanting to stay in 1973. Ostensibly, it's supposed to be Annie and this connection that he's created with her. That has certainly been implied here and there in big ways (Annie in the red dress, the hospital scene between the two) and smaller ways (Maria talking to both Annie and Sam about the two of them, etc).
From what I've read, the UK version delivered on that connection, and thus, pull. The problem with the US take is that I don't buy this great connection between *this* Annie and Sam. And based on what I've read, not many others do, and that's re-inforced based on what I haven't read; there's no buzz. I find a greater connection between Sam and Gene, Sam and Ray, Sam and Chris, hell, even Sam and Maria and Sam and Windy -- who's apparently disappeared, sigh. In fact, I honestly feel NO connection between Sam and Annie at all. No matter how much the show and actors try and sell it, the vast, vast majority of their scenes feel like two co-workers edging towards a tentative friendship because of their work relationship.
Jason O'Mara and Gretchen Moll just do not have a lick of chemistry. They're good actors, but there is no sizzle, no spark, nothing between them. And that was underscored in this episode when Moll had most of her scenes with Michael Imperoli's Ray because she and Imperoli had chemistry. Just as O'Mara has (had? :sniffsniff:) fabulous chemistry with Tanya Fischer's Windy, and he even has a nice -- if less, WHOAH! than with Fischer -- chemistry with Maggie Siff's Maria. Despite the fact that Sam and Annie are clearly supposed to be the rooting couple, O'Mara and Moll just don't have any chemistry or even that all-important connection with one another.
And that's really a shame, because this is a great show and I do believe that if it had had explosive chemistry between the actors playing Sam and Annie it might have stood a better shot at success. I honestly can't figure any other reason for its downturn in ratings. It started out so high (I think it was about 17 million), but just got lower even though the quality remained the same. There are certainly enough successful cop shows, procedurals and mind-bendy shows to believe that it has nothing to do with anything else involving the show.
Without that connection, there's no tension within the central conceit of the show that relies on that push and pull. There's no pull without the connection between Sam and Annie because the dialogue, direction and framing is clearly supposed to indicate that Annie is supposed to be the connection that provides that pull. Without any ounce of chemistry between Jason O'Mara and Gretchen Moll, there is no connection, thus ... no pull. No tension. So without that, when there are so many other options that provide that delicious television-watching-tension, people don't watch.