I'm both a session organizer and an applicant to a different panel. I received an acceptance for my panel a day or two ago (the fastest I've ever heard back), but haven't yet made decisions on the session I'm organizing.
Organizers aren't required to pass papers on to their second choice panel until Feb. 5, and we aren't required to list our final slate of panelists until Feb. 19th. My session got a pretty significant number of submissions, so it's taking a little while to read through our (all pretty good!) submissions and make cuts. And roundtable decisions (for people who don't get into the panel of their choice) aren't due to be finalized until early March.
My best guess about why the process takes so long is that ASA, unlike some other conferences I've been to, doesn't make it an all-or-nothing proposition where you apply as part of a pre-formed panel and are either accepted or rejected. Because you're allowed to list second choices and get sent on to a roundtable, each round of decisions often means a new person (or people) must read your paper. And given the seeming randomness of session popularity (some get barely enough submissions to stay afloat and will accept everyone; others get so many submissions they get granted a second session), I think the staged process is a decent solution.