Probably for the following reasons:
1. Neither Rice nor Powell kept their own home-brew email servers.
2. Neither Rice nor Powell withheld their work related emails for two years, and then still didn't turn over thousands of them.
3. Neither Rice nor Powell told Congress and the American people a string of lies about how those emails were handled.
4. Secretary of State Rice did not use email period.
https://www.youtube.com/watch?v=wbkS26PX4rc