1)Private clouds tend to use older technology than public clouds.
2)Public clouds shift capital expenses to operational expenses.
3)Public clouds have better utilization rates
4)Public clouds keep infrastructure costs low for new projects
5)Public clouds offer great elasticity
6)Public clouds get enterprises out of the “datacenter business”
7)Public clouds have greater economies of scale
8)Public clouds are hardened through continual hacking attempts.
9)Public clouds attract the best security people available
10)Private clouds suffer from “perimeter complacency.”
11)Private cloud staff competence is an unknown
12)Private cloud penetration testing is insufficient

You can go to the original article 12 Reasons Why Public Clouds Are Better Than Private Clouds” at ZDNet for more details. HT: Flexiant