Because on June 12, 2014, Palestinian Arab terrorists kidnapped and murdered three Israeli teens, Naftali Fraenkel, Gilad Shaer, and Eyal Yifrah, from the supposed territory of "Palestine" which they claim to be part of "Palestine."
Which is a war crime under the Rome Statute.
So Mahmoud Abbas is saying that the application should only be retroactive to June 13, when Israeli forces started to frantically look for the teens still hoped to be alive. Because he isn't interested in justice - but he is interested in protecting his Hamas partner from being indicted for a war crime that they brag about..
The cynicism is breathtaking.
Not that Mahmoud Abbas is alone in this cynical use of dates to frame Israel as being guilty of war crimes without context.
The exact same thing was done by William Schabas, chair of the UNHRC investigation into the events of last summer - a mandate that explicitly excludes the kidnapping of the three Israeli teens but that includes Israel's response to the kidnappings by choosing to only investigate events from June 13th. .