Because he's a nincompoop? Because all he cares about is his farkahkteh (that's Yiddish for really crappy) "legacy"? Because he's no fan of Israel, the Jewish state that's in the mullahs' nuclear crosshairs? Because he's a hard leftist who is hobbled--no, crippled--by a wonky worldview which abhors American puissance?
I think that about covers it.