Clinton cited a scheduling conflict as her reasoning for the drop out. However, a source close to the Clinton's told Slate that the decision to cancel was made because Kirstjen Nielsen, the former secretary of homeland security, is also scheduled to speak at the event.
Incidentally, one of the other speakers attending the summit is 2020 Democratic presidential candidate Tulsi Gabbard.
This news broke after Clinton and Gabbard's heated battle on Friday. Clinton suggested that Gabbard was being groomed by a foreign government for her presidential bid, referring to her as a "favorite of the Russians."