The big reason ACT gained marketshare is its partnerships with state ed departments–many states made the ACT mandatory at their schools (thanks to sweetheart deals from the ACT). Surprise surprise, now the SAT is doing the same and has already re-captured at least a few states.
Let’s not confuse this with students’ “choice”–it’s the state mandatory testing policies (and deals with the testing companies) that is actually driving the shifts in marketshare.