I can't really point to x, y, and z technical reasons. Several Pycons ago a few of us sprinted on SO support in Mailman and got pretty far. Ultimately though I had trouble getting it to do what I wanted, as easily as I wanted it to be. When I heard about SA, I pulled it down and started playing with it and it just seemed so much easier to build it into Mailman that I just stuck with it. What I really like is the brevity and yet power, but more importantly the minimal impact it forced on Mailman's current architecture. What this means for me is that I can move Mailman over to SA incrementally, allowing me to have more confidence things are working correctly, along with better milestones along the way. So SA seems quite nice and I haven't (yet?) hit any roadblocks.