What could be the reasons behind Trump firing Press Secy Katie in less than a week?

Katie Zacharia was fired as White House Press Secretary reportedly after her poorly received first briefing and a rough interview with Fox News' Sean Hannity, a Trump ally. The US President was reportedly "embarrassed" with the media coverage and advised her to reconsider the job. On Hannity's show, Zacharia admitted failing the bar exam but defended calling herself a lawyer.