1) She is counting votes behind the scenes. In the Senate, she can do this on her own. Collins needs to know two things. First, are the Senate GOP willing to live with leaving Obamacare in place? If not, then she, Cassidy and Murkowski hold all of the cards. They can hold firm in opposition to anything other than Collins-Cassidy, then CC is the only thing that can pass, and they win. Second, though, how many of them are unwilling to vote for Collins-Cassidy, for position-taking purposes, because it isn't conservative enough? This is the harder question. Some of them will worry about getting primaried if they don't stick with the most conservative "replace" option, even though they think CC a win on policy. Collins needs to count those votes. That's a behind-closed-doors process. (Usual disclaimer: fears of primaries are way overblown, but that doesn't mean the fears don't exist).
2) She needs to see everyone else's cards. Until there are other proposals, she is refraining from action because as long as her plan is the main one on the table, there just isn't much to do. When other plans are more formalized, she can act accordingly.
3) Collins sees things the way Boehner does. She doesn't think anything will happen, so why bother? If constituents press her on why she didn't try to repeal and replace Obamacare, she can say she was the first one in the Senate to offer a real alternative, and those layabouts in the Senate did what they always do. Lay about.
4) She has no plan. She's just making this up as she goes. Pro-tip for Sen. Collins if that's the case: hiding in a refrigerator won't save you if somebody tries to keep this going for too long and McConnell winds up using the nuclear option to try to pass something purer...
Why is Collins keeping quiet? I don't know. I'm just laying out some possibilities.
