There are multiple problems with that:
1. She sent it on purpose to someone she wasn't allowed to send it to. This is enough to get her sacked. The mistake was sending it to a third person, when she intended to send it to the wife of an MP (think about how dodgy that is for a start) and that's how she was found out.
2. She's the Home Secretary and deals with more serious confidential information than anyone else in government. She is not allowed to make mistakes like that, and there will be protocols and controls in place to stop it happening, which is why she had to deliberately send it to her personal email first, and then onto someone else.
She didn't make a mistake, she deliberately broke the rules (and I would assume the law), and by being careless when she was breaking them, got found out.