A wonderful possible answer to "why the cross?" when discussing the atonement.
"Yet [Paul's] most vivid instruction in this truth would have come from the cross of Christ itself. There, in the most humiliating off all forms of execution, Paul discovered the surpassing power of God (cf. 1 Corinthians 1:23-24). Moreover, he realised that it was especially in the cross that the power of God could dwell. Had it appeared in some other place arrogant humans would have been tempted to usurp it for themselves and to treat it as an object of personal boasting. In the first century the cross was simply too repugnant to be exploited for personal gain." ( Timothy B. Savage, Power through Weakness: Paul's Understanding of the Christian Ministry in 2 Corinthians (Society for New Testament Studies Monograph Series) (Cambridge University Press, 1995), 168.)