So...to answer directly.

TBB was in alpha at one point. So was OpenMP, MPI, ...

Just because you have an established library doesn't mean something else can't come along. Said another way, just because new doesn't mean to dismiss it. <rant> Do you have a quill pen? Hmm, didn't think so. If somebody came along and said: "Hey, look they both write, why use that fountain pen." Then we'd likely be writing with feathers today. </rant>