What evidence do you have that this was her reason? Because I'm sure you could prove it in a court of law.
Because the law is there for that reason.... to make sure our politicians can't pull a bunch of crap we can never know about.
The only way anyone can evade the law is to break it. In the FOIA-accessible inventory of American government information, everything will become accessible someday.
Obama and a lot of foreign folks had her "private" email and used it. For important business.