Why did elie leave his father when he hears the sirens

What chapter are you referring to?

If you're talking about the chapter I think you are, then it's because of his own fears, and his own weakness. Elie was not strong enough to try and help his after having been starved, beaten and tortured for years by Nazis. At that point, he was in survival mode, and as much as he cared about his father, his brain had one main focus: his own survival. Awkward if that's not the chapter you were referring to though lol.