User blog comment:Legionwrex/How did Saren reach the Citadel?/@comment-1440333-20130507094740/@comment-3217145-20130507152113

This, I think. Usually relays take a lot of calibrating and adjusting and waiting for targeting vectors and all that to get where you want. Shepard and co. barreled in on a huge hunk of steel and it sproinged them across the galaxy. I would assume that the Protheans wouldn't initially design something that would end with them being splattered across the wall when they used it.