It's that way because...
Syria is faced with a huge Iraqi refugees crisis.
Iran is Shi-ite and Iraq is in majority shi-ite too.
True, the Lebanese blame the anti-syrian MPs assassinations on Syria.
But that needs proof of a direct connection between the syrian government and the pro-syrian factions in Lebanon.
Same for Iran: where's the smoking gun the Iranian government is involved in sending Al Qaeda terrorists in Iraq?
Far too often, in both cases, the scenario runs like this: groups of uncontrollable terrorists do indeed provide help from Iran and Syria.
But just as the US troops found out about Iraq, those countries have huge uninhabited areas where it's very easy to hide.