It's a matter of jurisdiction, and arresting Assad for war crimes comes under the International Criminal Court!

Four signatory states—Israel, Sudan, the United States and Russia—have informed the UN Secretary General that they no longer intend to become states parties and, as such, have no legal obligations arising from their signature of the Statute.

You will notice that the US and Russia have no legal obligations arising from their signatures of the Statute! So, don't look for Assad to be tried for war crimes, at all!!!